smb3: if server does not support posix do not allow posix mount option

If user specifies "posix" on an SMB3.11 mount, then fail the mount
if server does not return the POSIX negotiate context indicating
support for posix.

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>