LU-12264 lnet: Protect lp_dc_pendq manipulation with lp_lock

LU-12264 lnet: Protect lp_dc_pendq manipulation with lp_lock

Protect the peer discovery queue from concurrent manipulation by
acquiring the lp_lock.

Lustre-change: https://review.whamcloud.com/34798
Lustre-commit: dd16a31bf4ae874a69cc7dc5fe1f3197993630ae

Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: If43b877c1c7ea203f346a3d6ea846f00b8f9661f
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36037
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>