net: don't use input buffer of __dev_alloc_name() as a scratch space

Callers of __dev_alloc_name() want to pass dev->name as
the output buffer. Make __dev_alloc_name() not clobber
that buffer on failure, and remove the workarounds
in callers.

dev_alloc_name_ns() is now completely unnecessary.

The extra strscpy() added here will be gone by the end
of the patch series.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231023152346.3639749-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>