[CIFS] remove unknown mount option warning message

Jeff's previous patch which removed the unneeded rw/ro
parsing can cause a minor warning in dmesg (about the
unknown rw or ro mount option) at mount time. This
patch makes cifs ignore them in kernel to remove the warning
(they are already handled in the mount helper and VFS).

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