cifs: fix wsize negotiation to respect max buffer size and active signing (try #4)

Hopefully last version. Base signing check on CAP_UNIX instead of
tcon->unix_ext, also clean up the comments a bit more.

According to Hongwei Sun's blog posting here:

    http://blogs.msdn.com/b/openspecification/archive/2009/04/10/smb-maximum-transmit-buffer-size-and-performance-tuning.aspx



CAP_LARGE_WRITEX is ignored when signing is active. Also, the maximum
size for a write without CAP_LARGE_WRITEX should be the maxBuf that
the server sent in the NEGOTIATE request.

Fix the wsize negotiation to take this into account. While we're at it,
alter the other wsize definitions to use sizeof(WRITE_REQ) to allow for
slightly larger amounts of data to potentially be written per request.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>