EN-16:17: virtual memory issues.

Due to increased parallelism and optimizations in several parts of the
system, the previously latent bugs in VM become much easier to trigger,
affecting a significant number of the FreeBSD users. The exact technical
details of the issues are provided in the commit messages of the merged
revisions, which are listed below with short summaries.

r301184 prevent parallel object collapses, fixes object lifecycle
r301436 do not leak the vm object lock, fixes overcommit disable
r302243 avoid the active object marking for vm.vmtotal sysctl, fixes

"vodead" hangs

r302513 vm_fault() race with the vm_object_collapse(), fixes spurious

SIGSEGV

r303291 postpone BO_DEAD, fixes panic on fast vnode reclaim

Approved by: so