f2fs: Simplify SEEK_DATA implementation

Instead of finding the first dirty page and then seeing if it matches
the index of a block that is NEW_ADDR, delay the lookup of the dirty
bit until we've actually found a block that's NEW_ADDR.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>