btrfs: initialize location to fix -Wmaybe-uninitialized in btrfs_lookup_dentry()

Some arch + compiler combinations report a potentially unused variable
location in btrfs_lookup_dentry(). This is a false alert as the variable
is passed by value and always valid or there's an error. The compilers
cannot probably reason about that although btrfs_inode_by_name() is in
the same file.

   >  + /kisskb/src/fs/btrfs/inode.c: error: 'location.objectid' may be used
   +uninitialized in this function [-Werror=maybe-uninitialized]:  => 5603:9
   >  + /kisskb/src/fs/btrfs/inode.c: error: 'location.type' may be used
   +uninitialized in this function [-Werror=maybe-uninitialized]:  => 5674:5

   m68k-gcc8/m68k-allmodconfig
   mips-gcc8/mips-allmodconfig
   powerpc-gcc5/powerpc-all{mod,yes}config
   powerpc-gcc5/ppc64_defconfig

Initialize it to zero, this should fix the warnings and won't change the
behaviour as btrfs_inode_by_name() accepts only a root or inode item
types, otherwise returns an error.

Reported-by: Geert Uytterhoeven <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: kdave

fs/btrfs/inode.c

@@ -5664,7 +5664,7 @@ struct inode *btrfs_lookup_dentry(struct inode *dir, struct dentry *dentry)
 	struct inode *inode;
 	struct btrfs_root *root = BTRFS_I(dir)->root;
 	struct btrfs_root *sub_root = root;
-	struct btrfs_key location;
+	struct btrfs_key location = { 0 };
 	u8 di_type = 0;
 	int ret = 0;