geli init with multiple providers - fix init and fix -B "none"

geli init with multiple providers - fix init and fix -B "none"

Apply some fixes post rS336659, which allowed multiple provders to be
initialized in a single command.
- Fix issue where second and subsequent providers would fail init.
  This was due to the metadata struct being zeroed after the first
  provider init was completed, despite containing common data required
  for subsequent providers.
- Fix issue where -B "none" would still result in the metadata being
  backed-up if multiple providers had been specified. This was due to
  the backupfile of "none" being incorrectly made unique for each
  provider by appending "-<prov>".

Approved by:	asomers
Approved by:	re (gjb)
Differential Revision:	https://reviews.freebsd.org/D17096