SUNRPC: Add errno-to-GSS status conversion helper

SUNRPC: Add errno-to-GSS status conversion helper

The crypto/krb5 library returns standard negative errno values,
but the GSS mechanism layer reports results as GSS_S_* major
status codes. A translation is needed at each call site that
will be switched to the new library.

Rather than open-coding the mapping in every wrapper, provide a
single helper function.

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>