staging/lustre/lnet: Fix use after free in ksocknal_send

Call to ksocknal_launch_packet might schedule a callback that
might free the just sent message, and so subsequent access to it
via lntmsg->msg_vmflush goes to freed memory.

Instead we'll just remember if we are in the vmflush thread and
only restore if we happened to set mempressure flag.

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/8667
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4360


Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>