timecounter: Lock the timecounter list

timecounter: Lock the timecounter list

Timecounter registration is dynamic, i.e., there is no requirement that
timecounters must be registered during single-threaded boot.  Loadable
drivers may in principle register timecounters (which can be switched to
automatically).  Timecounters cannot be unregistered, though this could
be implemented.

Registered timecounters belong to a global linked list.  Add a mutex to
synchronize insertions and the traversals done by (mpsafe) sysctl
handlers.  No functional change intended.

Reviewed by:	imp, kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32511