kernel - Remove last exclusive vnode vm_object lock from the critical path

kernel - Remove last exclusive vnode vm_object lock from the critical path

* Remove the last exclusive vnode vm_object lock from the critical path.

* Path gets hit on exit, but it matters a lot if one is fork/exec'ing a
  lot of binaries.  For example, builds which fork/exec huge numbers of
  concurrent /bin/sh's, compilers, and other programs.

* vfork/exec rate on blade server, 10000 x 8-threads (80000 total) reduced
  from 10.6 seconds to 3.8 seconds, for a major 2.7x improvement in
  performance.