xfs: rename xfs_bmap_is_real_extent to is_written_extent

The name of this predicate is a little misleading -- it decides if the
extent mapping is allocated and written.  Change the name to be more
direct, as we're going to add a new predicate in the next patch.

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