btrfs: open code inexact rbtree search in tree_search

The call chain from

tree_search
  tree_search_for_insert
    __etree_search

can be open coded and allow further simplifications, here we need a tree
search with fallback to the next node in case it's not found. This is
represented as __etree_search parameters next_ret=valid, prev_ret=NULL.

Signed-off-by: David Sterba <dsterba@suse.com>