LU-11296 osc: speed up page cache cleanup during blocking ASTs

LU-11296 osc: speed up page cache cleanup during blocking ASTs

While we are cleaning a write lock, we don't need to check if
page cache pages under this lock are covered by another lock.

If a client needs to give up its lock, cleaning gigabytes of
page cache can take quite a long time.

Signed-off-by: Andrew Perepechko <c17827@cray.com>
Cray-bug-id: LUS-6352
Change-Id: I576130216ed4de4e352ea697bddb5ff83046443a
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-on: https://review.whamcloud.com/33090
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>