Per-mount 'config' object

With support for multiple mounts of devpts, the 'config' structure really
represents per-mount options rather than config parameters. Rename 'config'
structure to 'pts_mount_opts' and store it in the super-block.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>