fsx: new option to disable msync(MS_SYNC) after each write via mmaped region

This option should be useful for testing if a filesystem uses the
unified buffer / page cache.
Or, if filesystem's emulation of the unified cache works as expected.
This should be the case for e.g. ZFS.

MFC after:	1 week
fsx: add an option to randomly call msync(MS_INVALIDATE)

This call should be a sufficiently close approximation of what happens
when a filesystem is unmounted and remounted.  To be more specific, it
should test that the data that was in the page cache is the same data
that ends up on a stable storage or in a filesystem's internal cache,
if any.
This will catch the cases where a page with modified data is marked as
a clean page for whatever reason.

While there, make logging of the special events (open+close before
plus invalidation now) more generic and slightly better than the previous
hack.

MFC after:	10 days
zfs page_busy: fix the boundaries of the cleared range

This is a fix for a regression introduced in r246293.

vm_page_clear_dirty expects the range to have DEV_BSIZE aligned boundaries,
otherwise it extends them.  Thus it can happen that the whole page is
marked clean while actually having some small dirty region(s).
This commit makes the range properly aligned and ensures that only
the clean data is marked as such.

It would interesting to evaluate how much benefit clearing with DEV_BSIZE
granularity produces.  Perhaps instead we should clear the whole page
when it is completely overwritten and don't bother clearing any bits
if only a portion a page is written.

Reported by:	George Hartzell <hartzell@alerce.com>,
		Richard Todd <rmtodd@servalan.servalan.com>
Tested by:	George Hartzell <hartzell@alerce.com>,
Reviewed by:	kib
MFC after:	5 days
taskqueue_cancel: garbage collect a write-only variable

MFC after:	3 days