generic: test idmapped mount circular mappings

generic: test idmapped mount circular mappings

This is a regression test for the fix in [1]. The test makes sure that
setattr behaves correctly on idmapped mounts that make use of circular
mappings. Such mappings may e.g. be used to allow two users to share
home directories through the same idmapped mount. The tests are
explained in detail in code comments.

[1]: commit 968219708108 ("fs: handle circular mappings correctly")
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: fstests@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>