f2fs: reflow prepare_write_begin

Reflow prepare_write_begin so that it reads more straight forward,
and so that there is one place that does an extent cache lookup
instead of three, two of which are hidden in f2fs_get_block calls.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>