log: fix locking typo/stupid for dump_recent()

We weren't locking m_flush_mutex properly, which in turn was leading to
racing threads calling dump_recent() and garbling the crash dump output.

Backport: bobtail, argonaut
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>