[AX.25]: Fix unchecked ax25_linkfail_register uses

ax25_linkfail_register 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>