LU-7330 ldlm: fix race of starting bl threads

LU-7330 ldlm: fix race of starting bl threads

There is race in the code of starting bl threads which leads to
thread number exceeds the maximum number when race happened, it
can also lead to duplicated thread name.

This patch fixes the race and cleanup the code a bit.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9c9be125d1d76890b8c52476684976dad3cb3d87
Reviewed-on: http://review.whamcloud.com/17026
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>