Parse mount options just once and copy them to super block

Since all the mount option parsing is done in devpts, we could do it
just once and pass it around in devpts functions and eventually store
it in the super block.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>