kernel: Add a final p_lock wait loop before kfree'ing the process

kernel: Add a final p_lock wait loop before kfree'ing the process

Since p_lock can be acquired during the process removal, there
needs to be a final wait loop after the proc has been removed
from all queues, just before the kfree(p).  It will be a very
rare case but it definitely can still occur if e.g. the SYSCTL_OUT
code blocks on a VM fault or something like that.

Requested-by: dillon@
Dragonfly-bug: <http://bugs.dragonflybsd.org/issue1996>