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

fs/afs/super.c

@@ -407,8 +407,10 @@ static int afs_validate_fc(struct fs_context *fc)
 			return PTR_ERR(volume);
 
 		ctx->volume = volume;
-		if (volume->type != AFSVL_RWVOL)
+		if (volume->type != AFSVL_RWVOL) {
 			ctx->flock_mode = afs_flock_mode_local;
+			fc->sb_flags |= SB_RDONLY;
+		}
 	}
 
 	return 0;