LU-12991 lnet: fix rspt counter

LU-12991 lnet: fix rspt counter

rsp entries must freed via lnet_rspt_free function to avoid counter
leak. handle NULL allocation properly.

Test-parameters: trivial

Cray-bug-id: LUS-8189
Change-Id: I7630d375387593e28bfbe2c4a3ea3712a239f64f
Signed-off-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-on: https://review.whamcloud.com/36895
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>