erofs-utils: mkfs: support inline xattr reservation for rootdirs

erofs-utils: mkfs: support inline xattr reservation for rootdirs

Due to the current on-disk limitation (16-bit on-disk root_nid), on-disk
root inodes must be updated in place for now.

If rootdir xattr sizes are expanded during incremental updates, there
may be insufficient space to keep additional extended attributes.

To work around this, let's add a mkfs option `--root-xattr-isize=#` to
specify the minimum inline xattr size of root directories in advance.

Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240729075027.712339-2-hsiangkao@linux.alibaba.com