[CIFS] Fix SMB2 negotiation support to select only one dialect (based on vers=)

Based on whether the user (on mount command) chooses:

vers=3.0 (for smb3.0 support)
vers=2.1 (for smb2.1 support)
or (with subsequent patch, which will allow SMB2 support)
vers=2.0 (for original smb2.02 dialect support)

send only one dialect at a time during negotiate (we
had been sending a list).

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>