LU-2263 ptlrpc: deadlock of obd_import::imp_lock

ptlrpc_at_recv_early_reply() calls ptlrpc_at_set_req_timeout() with
hold of ptlrpc_request::rq_lock, and
ptlrpc_at_set_req_timeout()->import_at_get_index() requires imp_lock,
it violates locking-order rule of Lustre, which assumes rq_lock can
nest in imp_lock.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I924839b59ca1fa1cdf01568872de867ce3985f8b
Reviewed-on: http://review.whamcloud.com/4880
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>