btrfs: pass the extent buffer for the btrfs_item_nr helpers

This is actually a change for extent tree v2, but it exists in
btrfs-progs but not in the kernel.  This makes it annoying to sync
accessors.h with btrfs-progs, and since this is the way I need it for
extent-tree v2 simply update these helpers to take the extent buffer in
order to make syncing possible now, and make the extent tree v2 stuff
easier moving forward.

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