neighbor: tracing: Have neigh_create event use __string()

The dev field of the neigh_create event uses __dynamic_array() with a
fixed size, which defeats the purpose of __dynamic_array(). Looking at the
logic, as it already uses __assign_str(), just use the same logic in
__string to create the size needed. It appears that because "dev" can be
NULL, it needs the check. But __string() can have the same checks as
__assign_str() so use them there too.

Link: https://lkml.kernel.org/r/20220705183741.35387e3f@rorschach.local.home



Cc: David Ahern <dsahern@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>