mkfs.f2fs: expand scalability of nat bitmap

mkfs.f2fs: expand scalability of nat bitmap

Previously, our total node number (nat_bitmap) and total nat segment count
will not monotonously increase along with image size, and max nat_bitmap size
is limited by "CHECKSUM_OFFSET - sizeof(struct f2fs_checkpoint) + 1", it is
with bad scalability when user wants to create more inode/node in larger image.

So this patch tries to relieve the limitation, by default, limitting total nat
entry number with 20% of total block number.

Before:
image_size(GB)	nat_bitmap	sit_bitmap	nat_segment	sit_segment
16		3836		64		36		2
32		3836		64		72		2
64		3772		128		116		4
128		3708		192		114		6
256		3580		320		110		10
512		3260		640		100		20
1024		2684		1216		82		38
2048		1468		2432		44		76
4096		3900		4800		120		150

After:
image_size(GB)	nat_bitmap	sit_bitmap	nat_segment	sit_segment
16		256		64		8		2
32		512		64		16		2
64		960		128		30		4
128		1856		192		58		6
256		3712		320		116		10
512		7424		640		232		20
1024		14787		1216		462		38
2048		29504		2432		922		76
4096		59008		4800		1844		150

Add a new option '-i' to control turning on/off this feature.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>