gfs2: Silence possible null pointer dereference warning

In gfs2_rbm_find, rs is always NULL when minext is NULL, so
gfs2_reservation_check_and_update will never be called on a NULL minext.
This isn't innediately obvious though, so also check for a NULL minext
for better code readability.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>