fuse: drop idmap argument from __fuse_get_acl

We don't need to have idmap in the __fuse_get_acl as we don't
have any use for it.

In the current POSIX ACL implementation, idmapped mounts are
taken into account on the userspace/kernel border
(see vfs_set_acl_idmapped_mnt() and vfs_posix_acl_to_xattr()).

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>