KERNEL_ASSERT_LOCKED(9): Assertion for kernel lock (Rev. 3)

KERNEL_ASSERT_LOCKED(9): Assertion for kernel lock (Rev. 3)

This adds a new assertion macro, KERNEL_ASSERT_LOCKED(), to assert that
kernel_lock is held.  In the long process of removing kernel_lock, there will
be a lot (hundreds or thousands) of use of this; virtually almost all functions
in !MP-safe subsystems should have this assertion.  Thus this assertion should
have a short, good name.

Not only that "KERNEL_ASSERT_LOCKED" is consistent with other KERNEL_* and
SCHED_ASSERT_LOCKED() macros.

Input from dlg@ guenther@ kettenis@.

OK dlg@ guenther@