mroute: partially sanitize the file

[mjg]

Summary

There is rampant inconsitent formatting all around, make it mostly
style(9)-conformant.

While here:
- drop malloc casts
- rename a rw lock from mroute_mtx to mroute_lock
- replace NOTREACHED comment with __assert_unreachable

there are some bugs i'm going to need to fix some time later in that file, sounds like a good time to get the above out of the way