Revert "net: core: maybe return -EEXIST in __dev_alloc_name"

This reverts commit d6f295e9; some userspace (in the case
we noticed it's wpa_supplicant), is relying on the current
error code to determine that a fixed name interface already
exists.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>