f2fs-tools: support small RO partition

f2fs-tools: support small RO partition

This patch adds a readonly feature on small partitions by eliminating
on-disk layout for data writes such as provisioning/reserved space and
SSA.

This requires f2fs updates to understand RO feature in superblock.

Tested 4 cases:
 sload.f2fs -c -a lz0 -f kernel/fs $DEV
 sload.f2fs -c -a lz4 -f kernel/fs $DEV
 sload.f2fs -c -r -a lz4 -f kernel/fs $DEV
 sload.f2fs -c -L 3 -r -a lz4 -f kernel/fs $DEV

after:
 mkfs.f2fs -O ro,compression,extra_attr -f $DEV
 fsck.f2fs $DEV

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