vfs: Add tmpfs tests for idmap mounts

vfs: Add tmpfs tests for idmap mounts

This patch calls all tests in the suite s_idmapped_mounts, but with a
tmpfs directory mounted inside a userns. This directory is setup as the
mount point for the test that runs nested.

This excercises that tmpfs mounted inside a userns works as expected
regarding idmap mounts.

Signed-off-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>