SMB3.1.1: fix mount failure to some servers when compression enabled

When sending the compression context to some servers, they rejected
the SMB3.1.1 negotiate protocol because they expect the compression
context to have a data length of a multiple of 8.

Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>