Currently the ntptime code and resettodr() are Giant-locked. In
particular, the Giant is supposed to protect against parallel
ntp_adjtime(2) invocations. But, for instance, sys_ntp_adjtime() does
copyout(9) under Giant and then examines time_status to return syscall
result. Since copyout(9) could sleep, the syscall result might be
inconsistent.

Another and more important issue is that if PPS is configured,
hardpps(9) is executed without any protection against the parallel
top-level code invocation. Potentially, this may result in the
inconsistent state of the ntptime state variables, but I cannot say
how serious such distortion is. The non-functional splclock() call in
sys_ntp_adjtime() protected against clock interrupts calling hardpps()
in the pre-SMP era.

Modernize the locking. A mutex protects ntptime data. Due to the
hardpps() KPI legitimately serving from the interrupt filters (and
e.g. uart(4) does call it from filter), the lock cannot be sleepable
mutex if PPS_SYNC is defined. Otherwise, use normal sleepable mutex
to reduce interrupt latency.

Reviewed by: imp, jhb
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
Differential revision: https://reviews.freebsd.org/D6825