Fix process exit vs reap race introduced in r329449

The race manifested itself mostly in terms of crashes with "spin lock
held too long".

Relevant parts of respective code paths:

exit: reap:
PROC_LOCK(p);
PROC_SLOCK(p);
p->p_state == PRS_ZOMBIE
PROC_UNLOCK(p);

				PROC_LOCK(p);

/* exit work */

				if (p->p_state == PRS_ZOMBIE) /* true */
					proc_reap()
					free proc

/* more exit work */
PROC_SUNLOCK(p);

Thus a still exiting process is reaped.

Prior to the change the zombie check was followed by slock/sunlock trip
which prevented the problem.

Even code prior to this commit has a bug: the proc is still accessed for
statistic collection purposes. However, the severity is rather small and
the bug may be fixed in a future commit.

Reported by: many
Tested by: allanjude