xfs: rename xfs_bmap_is_real_extent to is_written_extent

commit 877f58f53684f14ca3202640f70592bf44890924 upstream.

[ Slightly modify fs/xfs/libxfs/xfs_rtbitmap.c & fs/xfs/xfs_reflink.c to
  resolve merge conflict ]

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>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>