LU-9918 lnet: decref on peer after use

LU-9918 lnet: decref on peer after use

After looking up the peer for both ping and discover
we need to decref the peer so we don't lose a reference
on it. This needs to be done while the mutex_lock is held
to ensure the peer list remains stable.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: Ic57e67d21b8afe17a239cc496621bc4abf681077
Reviewed-on: https://review.whamcloud.com/28722
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>