b=21574 PING_INTERVAL_SHORT should not postpone the next ping

b=21574 PING_INTERVAL_SHORT should not postpone the next ping

i=oleg
i=andrew

Most our tests run with obd_timeout=20s, so PING_INTERVAL=5s, while
PING_INTERVAL_SHORT=7s. ptlrpc_pinger_commit_expected() was actually
not intended to delay pings.
Although we would prefer to schedule the next ping after a
bit more than 5s (jbd commit time), using 5s instead of 7s
is not a big deal since we will have to only wait for 5
additional seconds in the worst case.