f2fs: fix overflow due to condition check order

In the last ilen case, i was already increased, resulting in accessing out-
of-boundary entry of do_replace and blkaddr.
Fix to check ilen first to exit the loop.

Fixes: 2aa8fbb9693020 ("f2fs: refactor __exchange_data_block for speed up")
Cc: stable@vger.kernel.org # 4.8+
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>