[PATCH] blkmtd: use clear_page_dirty()

SetPageDirty() and ClearPageDirty() are low-level thing which filesystems
shouldn't be using.  They bypass dirty page accounting.

Cc: David Woodhouse <dwmw2@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>