LU-11004 ptlrpc: Serialize procfs access to scp_hist_reqs using mutex

LU-11004 ptlrpc: Serialize procfs access to scp_hist_reqs using mutex

scp_hist_reqs list can be quite long thus a lot of
userland processes can waste CPU power in spinlock cycles.

Change-Id: Ic0fa7338569f9a19213a1dc31f5479c96a76d23a
Cray-bug-id: LUS-5833
Signed-off-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-on: https://review.whamcloud.com/32307
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>