selftests/filesystems: test completing a context from another user namespace

selftests/filesystems: test completing a context from another user namespace

fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor, so the task that issues
FSCONFIG_CMD_CREATE need not be the one that created the context.
mount_capable() authorizes that for a caller holding CAP_SYS_ADMIN in an
ancestor of fc->user_ns, which any unprivileged user has over a user
namespace it just created.

binfmt_misc and overlayfs used to WARN_ON() the mismatch. Add a test for
both. Also cover the handover within one user namespace. That is a
supported thing to do and has to keep working.

Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-3-4e987911a39a@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>