idmapped-mounts: add fs_allow_idmap() helper

idmapped-mounts: add fs_allow_idmap() helper

Move the check whether the underlying filesystem supports idmapped
mounts into a separate helper. We will use it in the following patch to
make it possible to always run all tests that don't require idmapped
mounts.

Link: https://lore.kernel.org/r/20220113132421.865002-1-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: fstests@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>