LU-14541 llite: Check vmpage in releasepage

LU-14541 llite: Check vmpage in releasepage

We cannot release a page if the vmpage reference count is
>1, otherwise we will detach a vmpage from Lustre when the
page is still referenced in the VM.

This creates a situation where page discard for lock
cancellation will not find the page, so we can get stale
data reads.

This re-introduces the LU-12587 issue where direct I/O on
a client falls back to buffered I/O if there are pages in
cache, since it cannot flush them.  This is annoying but
not a huge problem.

Fixes: e59f0c9a245f ("LU-12587 llite: don't check vmpage refcount in ll_releasepage()")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I3aa1cd7330f5e7d1ba2ddb0c12779aa22f3d70b7
Reviewed-on: https://review.whamcloud.com/47262
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>