acm.c: Fix ACM reference counting for client object in acm_free_req

acm_free_req function decremented ref. counter always
in the first item in global client array instead of
request's client. If more than one client were connected
to ACM at the same time ref. count never went to 0.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>