LU-4423 ptlrpc: make ptlrpc_last_xid an atomic64_t

LU-4423 ptlrpc: make ptlrpc_last_xid an atomic64_t

This variable is treated like an atomic64_t,
so change it's type and simplify the code.

Change-Id: I4b219342222fd784ac1d7dc17660feb816bacd57
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35510
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>