staging: lustre: obdclass: serialize lu_site purge

Umount process relies on lu_site_purge(-1) to purge all
objects before umount, however, if there happen to have a
cache shrinker which calls lu_site_purge(nr) in parallel,
some objects may still being freed by cache shrinker even
after the lu_site_purge(-1) called by umount done.

This can be simply fixed by serializing purge threads,
since it doesn't make any sense to have them in parallel.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5331
Reviewed-on: http://review.whamcloud.com/11099


Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>