common: allow to run all tests on idmapped mounts

common: allow to run all tests on idmapped mounts

In addition to the generic and filesystem-specific idmapped mount
testsuites that already exist upstream today add simple
infrastructure so any test can be run on idmapped mounts simply by
setting IDMAPPED_MOUNTS=true in the config file or section. The main
user for now will be overlay to verify it works correctly on
idmapped mounts.

Note that the infrastructure is completely generic so every
filesystem that supports idmapped mounts can simply run all of their
tests idmapped. But note that not all ways to create a mount have
been converted yet. That includes e.g. _dmthin_mount and direct
calls to _mount in various tests.

In addition, there will be corner-cases. For example, xfs doesn't
allow bulkstat on idmapped mounts because it is a filesystem wide
operation, i.e. you can retrieve information for any inode in the
filesystem so the operation cannot be scoped reasonably under a
single mount. So xfstests testing bulkstat will fail as it's
blocked. Similar for some btrfs ioctl()s.

While we could of course restrict this testmode to -overlay for
which we know things work correctly we should not do this. It would
mean that people won't start using it and so we won't see issues
unless someone sits down and goes through more than 1000 tests and
figures out for each individual one whether it needs to be skipped
or not.

So instead allow this mode but for all filesystems so people can
start running and reporting failures and we can fix them up or block
them as we detect them.

Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <fstests@vger.kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>