[SLIP]: Simplify sl_free_bufs()

We can avoid assignments to the local variable 'tmp' and
actually get rid of tmp alltogether in sl_free_bufs(). This patch does
that.  This is safe since both kfree() and slhc_free() handles NULL
pointers gracefully.

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>