Do not run pmclog_configure_log() without pmc_sx protection.

Do not run pmclog_configure_log() without pmc_sx protection.

The r195005 unlocked pmc_sx before calling into pmclog_configure_log()
to avoid the LOR, but it allows flush or closelog to run in parallel
with the configuration, causing many failure modes.

Revert r195005.  Pre-create the logging process, allowing it to run
after the set up succeeded, otherwise the process terminates itself.

Reported and tested by:	pho
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D12882