xfs: fix comment for start time value of inode with bigtime enabled

The 'ctime', 'mtime', and 'atime' for inode is the type of
'xfs_timestamp_t', which is a 64-bit type:

/* fs/xfs/libxfs/xfs_format.h begin */
typedef __be64 xfs_timestamp_t;
/* fs/xfs/libxfs/xfs_format.h end */

When the 'bigtime' feature is disabled, this 64-bit type is splitted
into two parts of 32-bit, one part is encoded for seconds since
1970-01-01 00:00:00 UTC, the other part is encoded for nanoseconds
above the seconds, this two parts are the type of
'xfs_legacy_timestamp' and the min and max time value of this type are
defined as macros 'XFS_LEGACY_TIME_MIN' and 'XFS_LEGACY_TIME_MAX':

/* fs/xfs/libxfs/xfs_format.h begin */
struct xfs_legacy_timestamp {
        __be32          t_sec;          /* timestamp seconds */
        __be32          t_nsec;         /* timestamp nanoseconds */
};
 #define XFS_LEGACY_TIME_MIN     ((int64_t)S32_MIN)
 #define XFS_LEGACY_TIME_MAX     ((int64_t)S32_MAX)
/* fs/xfs/libxfs/xfs_format.h end */
/* include/linux/limits.h begin */
 #define U32_MAX         ((u32)~0U)
 #define S32_MAX         ((s32)(U32_MAX >> 1))
 #define S32_MIN         ((s32)(-S32_MAX - 1))
/* include/linux/limits.h end */

'XFS_LEGACY_TIME_MIN' is the min time value of the
'xfs_legacy_timestamp', that is -(2^31) seconds relative to the
1970-01-01 00:00:00 UTC, it can be converted to human-friendly time
value by 'date' command:

/* command begin */
[root@~]# date --utc -d '@0' +'%Y-%m-%d %H:%M:%S'
1970-01-01 00:00:00
[root@~]# date --utc -d "@`echo '-(2^31)'|bc`" +'%Y-%m-%d %H:%M:%S'
1901-12-13 20:45:52
[root@~]#
/* command end */

When 'bigtime' feature is enabled, this 64-bit type becomes a 64-bit
nanoseconds counter, with the start time value is the min time value of
'xfs_legacy_timestamp'(start time means the value of 64-bit nanoseconds
counter is 0). We have already caculated the min time value of
'xfs_legacy_timestamp', that is 1901-12-13 20:45:52 UTC, but the comment
for the start time value of inode with 'bigtime' feature enabled writes
the value is 1901-12-31 20:45:52 UTC:

/* fs/xfs/libxfs/xfs_format.h begin */
/*
 * XFS Timestamps
 * ==============
 * When the bigtime feature is enabled, ondisk inode timestamps become an
 * unsigned 64-bit nanoseconds counter.  This means that the bigtime inode
 * timestamp epoch is the start of the classic timestamp range, which is
 * Dec 31 20:45:52 UTC 1901. ...
 ...
 */
/* fs/xfs/libxfs/xfs_format.h end */

That is a typo, and this patch corrects the typo, from 'Dec 31' to
'Dec 13'.

Suggested-by: Darrick J. Wong <[email protected]>
Signed-off-by: Xiaole He <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>

Author: hexiaole1994

fs/xfs/libxfs/xfs_format.h

@@ -704,7 +704,7 @@ struct xfs_agfl {
  * When the bigtime feature is enabled, ondisk inode timestamps become an
  * unsigned 64-bit nanoseconds counter.  This means that the bigtime inode
  * timestamp epoch is the start of the classic timestamp range, which is
- * Dec 31 20:45:52 UTC 1901.  Because the epochs are not the same, callers
+ * Dec 13 20:45:52 UTC 1901.  Because the epochs are not the same, callers
  * /must/ use the bigtime conversion functions when encoding and decoding raw
  * timestamps.
  */