create_inode: set xattrs to the root directory as well

create_inode: set xattrs to the root directory as well

populate_fs do copy the xattrs for all files and directories, but the
root directory is skipped and as a result its extended attributes aren't
set. This is an issue when using mkfs to build a full system image that
can be used with SElinux in enforcing mode without making any runtime
fix at first boot.

This patch adds logic to set the root directory's extended attributes.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>