cifs: enable signing flag in SMB header when server has it on

cifs_sign_smb only generates a signature if the correct Flags2 bit is
set. Make sure that it gets set correctly if we're sending an async
call.

This patch fixes:

    https://bugzilla.kernel.org/show_bug.cgi?id=28142

Reported-and-Tested-by: JG <jg@cms.ac>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>