f2fs: remove buffer_head which has 32bits limit

This patch removes buffer_head dependency when getting block addresses.
Light reported there's a 32bit issue in f2fs_fiemap where map_bh.b_size is
32bits while len is 64bits given by user. This will give wrong length to
f2fs_map_block.

Reported-by: Light Hsieh <Light.Hsieh@mediatek.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>