rmlock: Temporarily revert commit c84bb8cd771c

It appears to have introduced a regression on arm64, possibly due to the
fact that the pcpu pointer is reloaded outside of the critical section
in _rm_rlock(). Until this is resolved one way or another, let's
revert.

Reported by: Ronald Klop <ronald-lists@klop.ws>
Sponsored by: The FreeBSD Foundation