LU-1061 agl: cl_locks_prune() waits for the last user

LU-1061 agl: cl_locks_prune() waits for the last user

The AGL sponsor holds user reference count on the cl_lock before
triggering AGL RPC. The user reference count on the cl_lock will
be released by AGL RPC reply upcall. Such AGL mechanism conflict
with cl_locks_prune(), which requires no lock is in active using
when the last iput() called.

So the cl_locks_prune() should wait for the last user reference
count to be released by the enqueue reply upcall.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I8998c0a247448b1b6c6e99995c9d956b1666279b
Reviewed-on: http://review.whamcloud.com/2079
Tested-by: Hudson
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>