Fix reaping on process fd close broken after r329449

The only consumer of proc_reap other than proc_to_reap was not updated
to not PROC_SLOCK.

Reported by:    Juan Ramon Molina Menor <listjm club.fr>

sys/kern/sys_procdesc.c

@@ -398,7 +398,6 @@ procdesc_close(struct file *fp, struct thread *td)
 			 * process's reference to the process descriptor when it
 			 * calls back into procdesc_reap().
 			 */
-			PROC_SLOCK(p);
 			proc_reap(curthread, p, NULL, 0);
 		} else {
 			/*