cifs: untangle server->maxBuf and CIFSMaxBufSize

server->maxBuf is the maximum SMB size (including header) that the
server can handle. CIFSMaxBufSize is the maximum amount of data (sans
header) that the client can handle. Currently maxBuf is being capped at
CIFSMaxBufSize + the max headers size, and the two values are used
somewhat interchangeably in the code.

This makes little sense as these two values are not related at all.
Separate them and make sure the code uses the right values in the right
places.

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