crypto: gcm: accept assoclen=24 for eesp software path

eesp computes assoclen = ip_eesp_hdr(8) + peer_hdr-iv(8) = 16;
seqiv then adds 8, making the inner assoclen passed to rfc4106 = 24.
The hardware AES-NI path already handles any assoclen (check commented
out in c30a335b804f); extend the software path to match.

Fixes: crypto_rfc4106_encrypt err -22 on eesp output without AES-NI