cifs: fix setting SecurityFlags to true

If you try to set /proc/fs/cifs/SecurityFlags to 1 it
will set them to CIFSSEC_MUST_NTLMV2 which no longer is
relevant (the less secure ones like lanman have been removed
from cifs.ko) and is also missing some flags (like for
signing and encryption) and can even cause mount to fail,
so change this to set it to Kerberos in this case.

Also change the description of the SecurityFlags to remove mention
of flags which are no longer supported.

Cc: stable@vger.kernel.org
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>