writeback: fix time ordering of the per superblock dirty inode lists 7

This one fixes four bugs.

There are a few situation in there where writeback decides it is going to skip
over a blockdev inode on the kernel-internal blockdev superblock.  It
presently does this by moving the blockdev inode onto the tail of the blockdev
superblock's s_dirty.  But

a) this screws up s_dirty's reverse-time-orderedness and

b) refiling the blockdev for writeback in another 30 second is rude.  We
   should try again sooner than that.

Fix all this up by using redirty_head(): move the blockdev inode onto the head
of the blockdev superblock's s_dirty list for prompt writeback.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>