SUNRPC: Switch MIC token generation to crypto/krb5

SUNRPC: Switch MIC token generation to crypto/krb5

gss_krb5_get_mic_v2() currently computes the MIC checksum by
driving a crypto_ahash directly, calling gss_krb5_checksum()
with the message body and GSS token header. Replace this with
a call to crypto_krb5_get_mic(), which performs the same keyed
hash operation through the crypto/krb5 library.

RFC 4121 Section 4.2.4 specifies that the checksum covers the
message body followed by the token header. Because the
crypto/krb5 metadata parameter is hashed before the data, the
GSS header cannot be passed as metadata. Instead, the header
is appended to the scatterlist after the body data, producing
the correct hash input ordering without using the metadata
parameter.

The scatterlist layout is:
  [checksum_output | message_body | gss_header]

The first scatterlist entry points directly into the
token buffer, so the checksum is written in place.

A shared helper, gss_krb5_mic_build_sg(), is introduced in
gss_krb5_crypto.c to construct this scatterlist layout. The
helper handles overflow allocation and scatterlist chaining
for large xdr_buf page arrays. It is reused by the verify_mic
counterpart in the following commit.

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>