LU-1169 mgs: Fix race during new fsdb creation.

LU-1169 mgs: Fix race during new fsdb creation.

Lock fsdb_mutex until the fsdb is loaded from llogs.
It fixes race between loading data from llog into fsdb
and obtaining data form it.

Xyratex-bug-id: MRP-230
Signed-off-by: Andriy Skulysh <andriy_skulysh@xyratex.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Bruce Korb <Bruce_Korb@us.xyratex.com>
Change-Id: I8c29040a182f363e83e61e57d3e20756f40300ea
Reviewed-on: http://review.whamcloud.com/2251
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>