[AX.25]: Fix unchecked rose_add_loopback_neigh uses

rose_add_loopback_neigh uses kmalloc and the callers were ignoring the
error value.  Rewrite to let the caller deal with the allocation.  This
allows the use of static allocation of kmalloc use entirely.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>