Implement ext4l_mkdir() to create directories on ext4 filesystems.
The function parses the path to extract the parent directory and
basename, resolves the parent inode, checks for existing entries,
and calls the Linux ext4_mkdir() function to create the directory.
Hook ext4l_mkdir into the filesystem layer via the .mkdir callback
in fs_legacy.c, enabling the standard 'mkdir' command to work with
ext4l filesystems.
Add a unit test that verifies directory creation, duplicate detection
(-EEXIST), nested directory creation, and error handling for
non-existent parent directories (-ENOENT).
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
5ae10b6f46