LU-14644 vvp: wait for nrpages to be updated

LU-14644 vvp: wait for nrpages to be updated

truncate_inode_pages() says there still may be a page in a process
of deletion upon return. wait for another thread which is doing
__delete_from_page_cache() to get nrpages updated.

Lustre-change: https://review.whamcloud.com/43464
Lustre-commit: 7d5d004506650c3739898e70d72c9a86b8aeeb88

Signed-off-by: Vitaly Fertman <c17818@cray.com>
Change-Id: I165b3d0866efaf2eb7e977520ebba4ee831874ab
HPE-bug-id: LUS-8842
Reviewed-on: https://review.whamcloud.com/46948
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>