ksmbd: use vfs_llseek instead of dereferencing NULL

By not checking whether llseek is NULL, this might jump to NULL. Also,
it doesn't check FMODE_LSEEK. Fix this by using vfs_llseek(), which
always does the right thing.

Fixes: f441584 ("cifsd: add file operations")
Cc: [email protected]
Cc: [email protected]
Cc: Ronnie Sahlberg <[email protected]>
Cc: Hyunchul Lee <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Reviewed-by: Namjae Jeon <[email protected]>
Acked-by: Al Viro <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: Steve French <[email protected]>

Author: zx2c4

fs/ksmbd/vfs.c

@@ -1048,7 +1048,7 @@ int ksmbd_vfs_fqar_lseek(struct ksmbd_file *fp, loff_t start, loff_t length,
 	*out_count = 0;
 	end = start + length;
 	while (start < end && *out_count < in_count) {
-		extent_start = f->f_op->llseek(f, start, SEEK_DATA);
+		extent_start = vfs_llseek(f, start, SEEK_DATA);
 		if (extent_start < 0) {
 			if (extent_start != -ENXIO)
 				ret = (int)extent_start;
@@ -1058,7 +1058,7 @@ int ksmbd_vfs_fqar_lseek(struct ksmbd_file *fp, loff_t start, loff_t length,
 		if (extent_start >= end)
 			break;
 
-		extent_end = f->f_op->llseek(f, extent_start, SEEK_HOLE);
+		extent_end = vfs_llseek(f, extent_start, SEEK_HOLE);
 		if (extent_end < 0) {
 			if (extent_end != -ENXIO)
 				ret = (int)extent_end;