xfs: move xfs_do_force_shutdown() and kill xfs_rw.c

xfs_do_force_shutdown now is the only thing in xfs_rw.c. There is no
need to keep it in it's own file anymore, so move it to xfs_fsops.c
next to xfs_fs_goingdown() and kill xfs_rw.c.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>