f2fs: simplfy a field name in struct f2fs_extent,extent_info

Rename a filed name from 'blk_addr' to 'blk' in struct {f2fs_extent,extent_info}
as annotation of this field descripts its meaning well to us.

By this way, we can avoid long statement in code of following patches.

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