LU-6805 ptlrpc: use smp unsafe at_init only for initialization

LU-6805 ptlrpc: use smp unsafe at_init only for initialization

at_init() is not smp safe, so it is not supposed to be used anywhere
but in at initialization.
Add at_reinit() - safe version of at_init().

Xyratex-bug-id: MRP-2726
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Change-Id: I90f872bd217d1e8008f8d35bb4606d168c9bf2ad
Reviewed-on: http://review.whamcloud.com/15522
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Chris Horn <hornc@cray.com>