pseudofs: make dup name an error instead of panic

There are enough cases where the duplicate name is caused by dubious
hardware configuration. Stopping the whole system instead of failing a
driver, or even a diagnostic subsystem in a driver, is more
user-friendly.

Another issue right now is that the check is only present for the
INVARIANTS builds, silently accepting the request to create an entry
with the existing name on production builds.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D50669