xfs: xfs_dabuf_map should return ENOMEM when map allocation fails

If the xfs_buf_map array allocation in xfs_dabuf_map fails for whatever
reason, we bail out with error code zero.  This will confuse callers, so
make sure that we return ENOMEM.  Allocation failure should never happen
with the small size of the array, but code defensively anyway.

Fixes: 45feef8f

 ("xfs: refactor xfs_dabuf_map")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>