xfs: Check for extent overflow when removing dir entries

Directory entry removal must always succeed; Hence XFS does the
following during low disk space scenario:
1. Data/Free blocks linger until a future remove operation.
2. Dabtree blocks would be swapped with the last block in the leaf space
   and then the new last block will be unmapped.

This facility is reused during low inode extent count scenario i.e. this
commit causes xfs_bmap_del_extent_real() to return -ENOSPC error code so
that the above mentioned behaviour is exercised causing no change to the
directory's extent count.

Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>