Both cluster_rbuild() and cluster_wbuild() sometimes set the pages

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