D20584 requires a way to acquire rangelocks on two vnodes concurrently without the risk of deadlock.
To do this, an non-blocking variant of at least one of rangelock_rlock() and rangelock_wlock() is needed.
This patch adds both rangelock_rlock_nonblock() and rangelock_wlock_nonblock() to do this and also
vn_rangelock_rlock_nonblock() and vn_rangelock_wlock_nonblock() using the above.
Not sure if rangelock_rlock()/rangelock_wlock() is in any man page?