rgw: replace GCM nonce with salt-based key derivation

rgw: replace GCM nonce with salt-based key derivation

Move randomness from the GCM IV into key derivation. Each object
now gets a 32-byte random salt stored in RGW_ATTR_CRYPT_SALT, fed
into HMAC-SHA256 alongside bucket_id and object name to produce a
unique per-object key. The GCM IV is a deterministic counter from
the chunk position, which is safe because the key never repeats.

All GCM modes (SSE-C, SSE-KMS, SSE-S3, RGW-AUTO) now go through
derive_object_key() before any encrypt or decrypt operation.

Rename AES_GCM_NONCE_SIZE to AES_GCM_IV_SIZE across CryptoAccel
backends (isa-l, openssl, qat) to reflect what it actually is.

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>