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: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>