cifs.upcall: bump SPNEGO msg version number and don't reject old versions

cifs.upcall: bump SPNEGO msg version number and don't reject old versions

When we added the ability for the kernel to send sec=mskrb5 to the
upcall, we subtly broke old cifs.upcall versions that don't understand
it. Bump the spnego message version to 2 to make this clear. Also,
change cifs.upcall to not reject requests with a version that's lower
than the current one, and to send the reply with the same version that
the request sent. The idea is to try and keep cifs.upcall backward
compatible with old kernels.

Signed-off-by: Jeff Layton <jlayton@redhat.com>