net: core: eliminate dev_alloc_name{,_ns} code duplication

dev_alloc_name contained a BUG_ON(), which I moved to dev_alloc_name_ns;
the only other caller of that already has the same BUG_ON.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>