xfrm: check trunc_len in XFRMA_ALG_AUTH_TRUNC

Maximum trunc length is defined by MAX_AH_AUTH_LEN (in bytes)
and need to be checked when this value is set (in bits) by
the user. In ah4.c and ah6.c a BUG_ON() checks this condiftion.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>