librbd: unprotect any non-unprotected snapshot

Include snapshots in the UNPROTECTING state as well, which can occur
after an unprotect is interrupted.

Fixes: #4100
Backport: bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit fe283813b44a7c45def6768ea0788a3a0635957e)