xfrm: Remove unused xfrm_state from xfrm_state_check_space

The xfrm_state argument is unused in this function, so remove it.
Also the name xfrm_state_check_space does not really match what this
function does. It actually checks if we have enough head and tailroom
on the skb. So we rename the function to xfrm_skb_check_space.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>