__dev_alloc_name() handles both the sprintf and non-sprintf
target names. This complicates the code.

dev_prep_valid_name() already handles the non-sprintf case,
before calling __dev_alloc_name(), make the only other caller
also go thru dev_prep_valid_name(). This way we can drop
the non-sprintf handling in __dev_alloc_name() in one of
the next changes.

commit 55a5ec9b ("Revert "net: core: dev_get_valid_name is now the same as dev_alloc_name_ns"") and
commit 029b6d14 ("Revert "net: core: maybe return -EEXIST in __dev_alloc_name"")
tell us that we can't start returning -EEXIST from dev_alloc_name()
on name duplicates. Bite the bullet and pass the expected errno to
dev_prep_valid_name().

dev_prep_valid_name() must now propagate out the allocated id
for printf names.
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231023152346.3639749-3-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>