f2fs: clean up addrs_per_{inode,block}()

Introduce a new help addrs_per_page() to wrap common code
from addrs_per_inode() and addrs_per_block() for cleanup.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>