gfs2: Use resource group glock sharing

This patch takes advantage of the new glock holder sharing feature for
resource groups.  We have already introduced local resource group
locking in a previous patch, so competing accesses of local processes
are already under control.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>