Remove creation of plfs tmp directories in plfs.spec.

Since we are no longer providing plfsrc.example and putting it in as
/etc/plfsrc, there is no default plfs configuration. Therefore, it is
not necessary for the plfs rpm to do anything with the old plfs
directories.
Merge pull request #247 from dshrader/master

spec file updated to not create and remove default PLFS mount points and backends.
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
Merge pull request #248 from chuckcranor/mlogfix

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