Add non-blocking trylock variants for the rangelock functions.

A future patch that will add a Linux compatible copy_file_range(2) syscall
needs to be able to lock the byte ranges of two files concurrently.
To do this without a risk of deadlock, a non-blocking variant of
vn_rangelock_rlock() called vn_rangelock_tryrlock() was needed.
This patch adds this, along with vn_rangelock_trywlock(), in order to
do this.
The patch also adds a couple of comments, that I hope clarify how the
algorithm used in kern_rangelock.c works.

Reviewed by: kib, asomers (previous version)
Differential Revision: https://reviews.freebsd.org/D20645