libxfs: use a convenience variable instead of open-coding the fork

Use a convenience variable instead of open-coding the inode fork.
This isn't really needed for now, but will become important when we
add the copy-on-write fork later.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>