crypto: aead - Add common IV generation code

This patch adds some common IV generation code currently duplicated
by seqiv and echainiv.  For example, the setkey and setauthsize
functions are completely identical.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --cc crypto/aead.c
Simple merge

diff --cc include/crypto/internal/geniv.h
Simple merge