LU-12441 lnet: Detach rspt when md_threshold is infinite

LU-12441 lnet: Detach rspt when md_threshold is infinite

MDs for pings use the infinite threshold on MD operations.
As such they aren't normally unlinkable as determined by
lnet_md_unlinkable(). We can cover this case by checking whether the
refcount is zero and threshold is LNET_MD_THRESH_INF.

Cray-bug-id: LUS-7366
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I4b9499bc6327523f1e911ebc4514002511e42236
Reviewed-on: https://review.whamcloud.com/35452
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>