btrfs: allow idmapped INO_LOOKUP_USER ioctl

The INO_LOOKUP_USER is an unprivileged version of the INO_LOOKUP ioctl
and has the following restrictions. The main difference between the two
is that INO_LOOKUP is filesystem wide operation wheres INO_LOOKUP_USER
is scoped beneath the file descriptor passed with the ioctl.
Specifically, INO_LOOKUP_USER must adhere to the following restrictions:

- The caller must be privileged over each inode of each path component
  for the path they are trying to lookup.

- The path for the subvolume the caller is trying to lookup must be reachable
  from the inode associated with the file descriptor passed with the ioctl.

The second condition makes it possible to scope the lookup of the path
to the mount identified by the file descriptor passed with the ioctl.
This allows us to enable this ioctl on idmapped mounts.

Specifically, this is possible because all child subvolumes of a parent
subvolume are reachable when the parent subvolume is mounted. So if the
user had access to open the parent subvolume or has been given the fd
then they can lookup the path if they had access to it provided they
were privileged over each path component.

Note, the INO_LOOKUP_USER ioctl allows a user to learn the path and name
of a subvolume even though they would otherwise be restricted from doing
so via regular VFS-based lookup.

So think about a parent subvolume with multiple child subvolumes.
Someone could mount he parent subvolume and restrict access to the child
subvolumes by overmounting them with empty directories. At this point
the user can't traverse the child subvolumes and they can't open files
in the child subvolumes.  However, they can still learn the path of
child subvolumes as long as they have access to the parent subvolume by
using the INO_LOOKUP_USER ioctl.

The underlying assumption here is that it's ok that the lookup ioctls
can't really take mounts into account other than the original mount the
fd belongs to during lookup. Since this assumption is baked into the
original INO_LOOKUP_USER ioctl we can extend it to idmapped mounts.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>