Use the generation count of the pv list to work around LOR between
pmap lock and pv list lock, and use the shared locking on
pvh_global_lock in pmap_remove_write(), same as it was done for
pmap_ts_referenced().

Noted and reviewed by:	alc (previous version)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Revert r254501.  Instead, reuse the type stability of the struct pmap
which is the part of struct vmspace, allocated from UMA_ZONE_NOFREE
zone.  Initialize the pmap lock in the vmspace zone init function, and
remove pmap lock initialization and destruction from pmap_pinit() and
pmap_release().

Suggested and reviewed by:	alc (previous version)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Both cluster_rbuild() and cluster_wbuild() sometimes set the pages
shared busy without first draining the hard busy state.  Previously it
went unnoticed since VPO_BUSY and m->busy fields were distinct, and
vm_page_io_start() did not verified that the passed page has VPO_BUSY
flag cleared, but such page state is wrong.  New implementation is
more strict and catched this case.

Drain the busy state as needed, before calling vm_page_sbusy().

Tested by:	pho, jkim
Sponsored by:	The FreeBSD Foundation