Fix a bug in r302252.

Change ntpadj_lock to spinlock always, and rename stuff removing
ADJ/adj from the names. ntp_update_second() requires ntp_lock and is
called from the tc_windup(), so ntp_lock must be a spinlock. Add
missed lock to ntp_update_second().

Tested by: pho (as part of the whole patch)
Reviewed by: jhb (same)
Noted by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
X-Differential revision: https://reviews.freebsd.org/D7302