f2fs-tools: get rid of unneeded fields in on-disk inode

f2fs-tools: get rid of unneeded fields in on-disk inode

As Jaegeuk reminded:

Once user updates f2fs-tools which support new fields in inode layout,
but do keep the kernel which can not support those fields, it will cause
old f2fs fail to mount new image due to root_inode's i_extra_isize value
sanity check.

So if f2fs-tools doesn't enable feature which will use new fields of
inode, we don't need to expand i_extra_isize to include them, let's just
let i_extra_isize point to the end of last valid extra field's position.

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