btrfs-progs: make receive work inside of subvolumes

btrfs-progs: make receive work inside of subvolumes

Kind of a big feature of btrfs is being able to have a default subvol.  However
the receive code generates the paths to the subvols from the root of the fs,
even in the case of a default subvol.  So instead figure out if we're inside of
a subvol, either because we have a different default or we've chroot'ed and are
using -m.  Then strip this extra path off of the subvol we find so we can look
up our parent properly.  Thanks

Reported-by: Neil Horman <nhorman@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>