btrfs: move btrfs_swapfile_pin into volumes.h

This isn't a great spot for this, but one of the swapfile helper
functions is in volumes.c, so move the struct to volumes.h.  In the
future when we have better separation of code there will be a more
natural spot for this.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>