Monitor counter for adaptive_hash_searches_btree is disabled.
There is existing variable "btr_cur_n_non_sea" which counts number of
searches using the B-tree (without using AHI).

Fix the monitor to use correct type MONITOR_EXISTING|MONITOR_DEFAULT_ON

Reviewed-by: Marko Makela <marko.makela@oracle.com>
RB: 7501