They are widely used all over the kernel and aren't contained only to the locking code.
You are damn right. So looks like all other kernel related asserts in systm.h aren't covered by _KERNEL. Then I'd suggest to proceed as is, with original patch, where MPASSes were right after KASSERT definition.
Then with a new patch tested with separate tinderbox build I will push all kernel related assertions under _KERNEL, all at once.