f2fs: write only the pages in range during defragment

@lend of filemap_write_and_wait_range is supposed to be a "offset
in bytes where the range ends (inclusive)". Subtract 1 to avoid
writing an extra page.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>