ICP: gcm: Allocate hash subkey table separately

While evaluating other assembler implementations it turns out that
the precomputed hash subkey tables vary in size, from 8*16 bytes
(avx2/avx512) up to 48*16 bytes (avx512-vaes), depending on the
implementation.

To be able to handle the size differences later, allocate
gcm_Htable dynamically rather then having a fixed size array, and
adapt consumers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #11102