xfs: scrub: avoid uninitialized return code

The newly added xfs_scrub_da_btree_block() function has one code path
that returns the 'error' variable without initializing it first, as
shown by this compiler warning:

fs/xfs/scrub/dabtree.c: In function 'xfs_scrub_da_btree_block':
fs/xfs/scrub/dabtree.c:462:9: error: 'error' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Return zero since the caller will exit the scrub code if we don't produce a
buffer pointer.

Fixes: 7c4a07a4 ("xfs: scrub directory/attribute btrees")
Reported-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NBrian Foster <bfoster@redhat.com>

fs/xfs/scrub/dabtree.c

@@ -336,7 +336,7 @@ xfs_scrub_da_btree_block(
 	xfs_ino_t			owner;
 	int				*pmaxrecs;
 	struct xfs_da3_icnode_hdr	nodehdr;
-	int				error;
+	int				error = 0;
 
 	blk = &ds->state->path.blk[level];
 	ds->state->path.active = level + 1;