afs: Mark a superblock for an R/O or Backup volume as SB_RDONLY

Mark a superblock that is for for an R/O or Backup volume as SB_RDONLY when
mounting it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org