xfs: simplify di_flags2 inheritance in xfs_ialloc

di_flags2 is initialized to zero for v4 and earlier file systems.  This
means di_flags2 can only be non-zero for a v5 file systems, in which
case both the parent and child inodes can store the field.  Remove the
extra di_version check, and also remove the rather pointless local
di_flags2 variable while at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>