[CIFS] Make cifs default wsize match what we actually want to send (52K

typically - header + 13 pages).

Forgetting to set wsize on the mount command costs more than 10% on large
write (can be much more) so this makes a saner default.  We still shrink
this default smaller if server can not support it.

Signed-off-by: Steve French <sfrench@us.ibm.com>