gfs2: Fix glock rhashtable rcu bug

gfs2: Fix glock rhashtable rcu bug

Before commit e3c2502631 "GFS2: Use resizable hash table for glocks",
glocks were freed via call_rcu to allow reading the glock hashtable
locklessly using rcu.  This was then changed to free glocks immediately,
which made reading the glock hashtable unsafe.  Bring back the original
code for freeing glocks via call_rcu.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Cc: stable@vger.kernel.org # 4.3+