gmac_init_ctx() duplicates most of the code in gcm_int_ctx() while
it just needs to set its own IV length and AAD tag length.
Introduce gcm_init_ctx_impl() which handles the GCM and GMAC
differences while reusing the duplicated code.
While here, fix a flaw where the AVX implementation would accept a
context using a byte swapped key schedule which it could not
handle. Also constify the IV and AAD pointers passed to
gcm_init{,_avx}().
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes#14529
8d9752569b