net: dst: Make dst_destroy() static and return void.

Since commit 52df157f ("xfrm: take refcnt of dst when creating
struct xfrm_dst bundle") dst_destroy() returns only NULL and no caller
cares about the return value.
There are no in in-tree users of dst_destroy() outside of the file.

Make dst_destroy() static and return void.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240202163746.2489150-1-bigeasy@linutronix.de


Signed-off-by: Paolo Abeni <pabeni@redhat.com>