[XFRM]: alg_key_len should be unsigned to avoid integer divides

alg_key_len is currently defined as 'signed int'. This unfortunatly
leads to integer divides in several paths.

Converting it to unsigned is safe and saves 208 bytes of text on i386.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>