[Filestore] Filesystem destruction should fail not only if there are active sessions, but if there are orphan sessions as well

[qkrorlqr]

Here

nbs/cloud/filestore/libs/storage/tablet/tablet_state_sessions.cpp

Line 398 in 3db95bc

for (const auto& session: Impl->Sessions) {

we iterate only over active sessions. We should iterate over both active and orphan sessions. Since filesystem destruction logic uses DescribeSessions, it will automatically guarantee that oprhan sessions will prevent filesystem destruction.