Remove reference to <linux/btrfs.h>, and instead use <btrfs/ioctl.h> like we're supposed to (from btrfs-progs)

This fixes compilation issues when btrfs.h isn't available (because we just need the relevant structs, which for userspace programs are supposed to come from btrfs-progs instead of the kernel headers).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)