This patch fixes a filesystem corrupting bug, present in 2.5.41 through
2.5.44.  It can cause ext2 indirect blocks to not be written out.  A
fsck will fix it up.

Under heavy memory pressure a PF_MEMALLOC task attemtps to write out a
blockdev page whose buffers are already under writeback and which were
dirtied while under writeback.

The writepage call returns -EAGAIN but because the caller is
PF_MEMALLOC, the page was not being marked dirty again.

The page sits on mapping->clean_pages for ever and it not written out.

The fix is to mark that page dirty again for all callers, regardless of
PF_MEMALLOC state.