SUNRPC: Switch wrap token decryption to crypto/krb5

SUNRPC: Switch wrap token decryption to crypto/krb5

Replace the per-enctype .decrypt callbacks (gss_krb5_aes_decrypt
and krb5_etm_decrypt) with a single gss_krb5_aead_decrypt()
wrapper that delegates to crypto_krb5_decrypt().

The new wrapper builds a scatterlist covering the secured
region (confounder through checksum), passes it to the AEAD
decrypt operation, and derives the confounder and checksum
lengths from the data offset and length that
crypto_krb5_decrypt() reports. The caller's token header
verification and buffer adjustment logic is unchanged.

Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>