authpf needs /var/authpf to exist and be writable by group authpf.
The vm_page_trysbusy() should not fail when shared busy counter or
VPB_BIT_WAITERS flag were changed between reading of busy_lock and the
cas.  The vm_page_sbusy(), which is the only user of
vm_page_trysbusy() in the tree, panics on the failure, which in these
cases is transient and do not mean that the current page state
prevents sbusying.

Retry the operation inside vm_page_trysbusy() if cas failed, only
return a failure when VPB_BIT_SHARED is cleared.

Reported and tested by:	pho
Reviewed by:	attilio
Sponsored by:	The FreeBSD Foundation
The vm_pageout_flush() functions sbusies pages in the passed pages
run.  After that, the pager put method is called, usually translated
to VOP_WRITE().  For the filesystems which use buffer cache,
bufwrite() sbusies the buffer pages again, waiting for the xbusy state
to drain.  The later is done in vfs_drain_busy_pages(), which is
called with the buffer pages already sbusied (by vm_pageout_flush()).

Since vfs_drain_busy_pages() can only wait for one page at the time,
and during the wait, the object lock is dropped, previous pages in the
buffer must be protected from other threads busying them.  Up to the
moment, it was done by xbusying the pages, that is incompatible with
the sbusy state in the new implementation of busy.  Switch to sbusy.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation