[glusterd/snapshot] Glusterd crashes when a same command
eg snapshot create is fired simultaneously on a node

Cause: In glusterd_mgmt_v3_initiate_snap_phases() , the function
glusterd_mgmt_v3_post_validate() asserts on the NULL value of
req_dic. req_dic is not initialized as
glusterd_mgmt_v3_initiate_lockdown() is not able to acquire the lock
and comes to the "out" section, before initializing req_dic
(via glusterd_mgmt_v3_build_payload)

Fix: Call glusterd_mgmt_v3_post_validate() only if the lock is
acquired.

Change-Id: I7cb55b6c0013ad1c8bbb922a62c34aab097bafe9
BUG: 1090047
Signed-off-by: Joseph Fernandes <josferna@redhat.com>
Reviewed-on: http://review.gluster.org/7500
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>