posix_acl: Fix the type of sentinel in get_acl

The type should be struct posix_acl * instead of void *.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>