timekeeping: Audit clock adjustments

Emit an audit record whenever the system clock is changed (i.e. shifted
by a non-zero offset) by a syscall from userspace. The syscalls than can
(at the time of writing) trigger such record are:
  - settimeofday(2), stime(2), clock_settime(2) -- via
    do_settimeofday64()
  - adjtimex(2), clock_adjtime(2) -- via do_adjtimex()

The new records have type AUDIT_TIME_INJOFFSET and contain the following
fields:
  - sec -- the 'seconds' part of the offset
  - nsec -- the 'nanoseconds' part of the offset

Example record (time was shifted backwards by ~15.875 seconds):

type=TIME_INJOFFSET msg=audit(1530616049.652:13): sec=-16 nsec=124887145

The records of this type will be associated with the corresponding
syscall records.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
[PM: fixed a line width problem in __audit_tk_injoffset()]
Signed-off-by: Paul Moore <paul@paul-moore.com>