Lock proctree in around fill_kinfo_proc().

Proctree lock is needed for correct calculation and collection of the
job-control related data in kinfo_proc. There was even an XXX comment
about it.

Satisfy locking and lock ordering requirements by taking proctree lock
around pass over each bucket in proc_iterate(), and in sysctl_kern_proc()
and note_procstat_proc() for individual process reporting.

Reviewed by: jilles
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27871