net/hamradio/6pack: Fix the size of a sk_buff used in 'sp_bump()'

We 'allocate' 'count' bytes here. In fact, 'dev_alloc_skb' already add some
extra space for padding, so a bit more is allocated.

However, we use 1 byte for the KISS command, then copy 'count' bytes, so
count+1 bytes.

Explicitly allocate and use 1 more byte to be safe.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>