acl: make posix_acl_clone() available to overlayfs

The ovl_get_acl() function needs to alter the POSIX ACLs retrieved from the
lower filesystem. Instead of hand-rolling a overlayfs specific
posix_acl_clone() variant allow export it. It's not special and it's not deeply
internal anyway.

Link: https://lore.kernel.org/r/20220708090134.385160-3-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>