[PATCH] filemap_fdatawrite_range() api: clarify -end parameter

I had trouble understanding working out whether filemap_fdatawrite_range()'s
`end' parameter describes the last-byte-to-be-written or the last-plus-one.
Clarify that in comments.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>