Once the blockdev inode for /dev/ram0 is dirtied we have a memory-backed
inode on the blockdev superblock's s_dirty list.

sync_sb_inodes() sees the memory-backed inode on the superblock and assumes
that all the other inodes on the superblock are also memory-backed.  This is
not true for the blockdev superblock!  We forget to write out dirty pages
against the following blockdevs.

Fix this by just leaving the inode dirty and moving on to inspect the other
blockdev inodes on sb->s_io.

(This is a little inefficient: an alternative is to leave dirtied
memory-backed inodes on inode_in_use, so nobody ever even considers them for
writeout.  But that introduces an inconsistency and is a bit kludgey).