flush changes on LOCK_EX unlock

flush changes on LOCK_EX unlock

right now, flock with LOCK_EX, on unlock, triggers an async store.
make it sync, but also, make sure to ask to have any in-memory
data sent to us (e.g. VM_StoreAllSegments); unlike Solaris
VMSYNC_INVAL, we don't want to invalidate the pages, just get copies
written back.

LOCK_SH did not, does not and AFAICT should not trigger writes.

Change-Id: Id4a72f73b685b5566bb31f6f610f22d806899280
Reviewed-on: http://gerrit.openafs.org/1846
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>