ext4: detect directories in ext4fs_exists()

While fat_exists() reports directories and files as existing
ext4fs_exists() only recognizes files. This lead to errors
when using systemd-boot with an ext4 file-system.

Change ext4fs_exists() to find any type of inode:
files, directories, symbolic links.

Fixes: a1596438 ("ext4fs ls load support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>