SUNRPC: Add crypto/krb5 enctype lookup to krb5_ctx

SUNRPC: Add crypto/krb5 enctype lookup to krb5_ctx

Each krb5_ctx currently points to a gss_krb5_enctype, the
rpcsec_gss_krb5 module's own enctype descriptor. To begin
using the common crypto/krb5 library, store a pointer to the
corresponding struct krb5_enctype (from <crypto/krb5.h>) as
well.

The lookup is performed in gss_import_v2_context() immediately
after the existing gss_krb5_lookup_enctype() call. If
crypto_krb5_find_enctype() cannot find a matching enctype the
context import fails, ensuring the module never operates with
a partially-initialized krb5_ctx.

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>