LU-11290 ldlm: page discard speedup

Improving check_and_discard_cb, allowing to cache
negative result of dlm lock lookup and avoid
excessive osc_dlm_lock_at_pgoff() calls.

HPE-bug-id: LUS-6432
Change-Id: I253f944bf430b06d0e1a300d22e5d9b2e97412bf
Signed-off-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-on: https://review.whamcloud.com/39327
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@hpe.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>