gfs2: gfs2_glock_get: Wait on freeing glocks

Keep glocks in their hash table until they are freed instead of removing
them when their last reference is dropped.  This allows to wait for any
previous instances of a glock to go away in gfs2_glock_get before
creating a new glocks.

Special thanks to Andy Price for finding and fixing a problem which also
required us to delete the rcu_read_unlock from the error case in function
gfs2_glock_get.

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