LU-17103 lnet: use workqueue for lnd ping buffer updates

LU-17103 lnet: use workqueue for lnd ping buffer updates

Introduce workqueue for handling lnd-initiated ping buffer
update requests.

This is done to avoid the possibility of monitor thread
lock up waiting for the "old" ping buffer refcount to get
decremented during the update, while the message which
triggers the decrement is on the monitor thread's own queue
waiting to be processed.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-lnet env=ONLY="207 500",ONLY_REPEAT=50
Fixes: 7ac399c5 ("LU-16949 lnet: get monitor thread to update ping buffer")
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I5176581703e52f4adbfff417040bebcc2489b79e