btrfs: move eb offset helpers into extent_io.h

These are very specific to how the extent buffer is defined, so this
differs between btrfs-progs and the kernel.  Make things easier by
moving these helpers into extent_io.h so we don't have to worry about
this when syncing ctree.h.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>