btrfs: add eb to btrfs_node_key_ptr_offset

This is a change needed for extent tree v2, as we will be growing the
header size.  This exists in btrfs-progs currently, and not having it
makes syncing accessors.[ch] more problematic.  So make this change to
set us up for extent tree v2 and match what btrfs-progs does to make
syncing easier.

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