LU-11269 ptlrpc: request's counter in import

LU-11269 ptlrpc: request's counter in import

which is separate from imp_refcount as the latter can be
used for other purposes and it's hard to use to track
requests.

to verify the theory that imp_refcount should be checked.

Change-Id: I7c273a73e2b1bb43059c7ed003ee2b7d09273bfe
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/37722
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>