generic: add test for tmpfs POSIX ACLs

generic: add test for tmpfs POSIX ACLs

Add a regression test for commit 705191b03d50 ("fs: fix acl translation").
This tests whether setting POSIX ACLs on a tmpfs mounted in a
non-initial user and mount namespace works as expected.

Note, once again the idmapped mount testsuite is grossly misnamed at
this point. It has morphed into a full-blown generic vfs feature
testsuite.

Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Zorro Lang <zlang@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>