fix a concurrency problem with mlog when using the PLFS native API.

mlog_filter() uses mlog_xst.mlog_facs if mlog_xst.tag is set, but
there is a window of time where mlog_open() was setting mlog_xst.tag
before mlog_xst.mlog_facs was init'd.   attempt to close that window.
consider converting mlog lock to PTHREAD_MUTEX_INITIALIZER in the
future.

issue #246.
reported by Jingwang and Zhenhua via John Bent