mount_smbfs stuck in devdrn

Hi

I've noticed a serious problem with mount_smbfs again: it doesn't work,
it gets stuck in devdrn state. I've reported this before, and I think it
was solved at the time (IIRC, interaction with devfs or devd), but it's
back again.

I can reproduce this 100% on two machines running different recent
versions of 7-CURRENT, compiled without WITNESS and INVARIANTS. It works
in 6-STABLE (maybe it was fixed in RELENG_6 only?).

I've filed a PR about this but I didn't get a confirmation mail so maybe
it's eaten by gnats again.

I'm using mount_smbfs like this:

# mount_smbfs -I host //ivoras_at_host/c$ /mnt