btrfs: use next_state/prev_state in merge_state

We use rb_next/rb_prev and then get the entry for the adjacent items in
an extent io tree.  We have helpers for this, so convert merge_state to
use next_state/prev_state and simplify the code.

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