gfs2: add compat_ioctl support

Out of the four ioctl commands supported on gfs2, only FITRIM
works in compat mode.

Add a proper handler based on the ext4 implementation.

Fixes: 6ddc5c3d ("gfs2: getlabel support")
Reviewed-by: Bob Peterson <rpeterso@redhat.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>