cifs: negotiates the highest SMB2+ version supported by default

cifs: negotiates the highest SMB2+ version supported by default

instead of hardcoding it to a potential outdated value.

For `smbclient` we only set max-protocol version and that could only
be smb2 or smb3 (no finer granularity) any how, so this was not
really correct.

Nowadays the kernel dropped SMB1 and tries to go for SMB2.1 or higher
by default, depending on what client and server supports. SMB2.1 is
Windows 7/2008R2 - both EOL since quite a bit, so ok as default lower
boundary.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>