Synchronize page laundering with pmap_extract_and_hold().

Before r207410, the hold count of a page in a page queue was protected
by the queue lock, and, before laundering a page, the page daemon
removed managed writeable mappings of the page before releasing the
queue lock. This ensured that other threads could not concurrently
create transient writeable mappings using pmap_extract_and_hold() on a
user map, as is done for example by vmapbuf(). With that revision,
however, a race can allow the creation of such a mapping, meaning that
the page might be modified as it is being laundered, potentially
resulting in it being marked clean when its contents do not match
those given to the pager. Close the race by using the page lock to
synchronize the hold count check in vm_pageout_cluster() with the
removal of writeable managed mappings.

Reported by: alc
Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D12084