Remove reference to <linux/btrfs.h>, and instead copy "ioctl.h" from btrfs-progs for btrfs structs

btrfs-progs doesn't "#include <linux/btrfs.h>" anywhere because that's a kernel header, not something for userspace.

This fixes compilation issues when btrfs.h isn't available (because we just need the relevant structs, and this ioctl.h file is literally a copy from btrfs-progs with a single comment added to the top to reference the source of the file itself)

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