Avoid repeated calls to btr_get_search_latch()

The MySQL 5.7 implementation of the parameter adaptive_hash_index_parts introduced a large number of calls to the function btr_get_search_latch(). The adaptive hash index bucket of a particular B-tree index is constant; therefore we should avoid recomputing the hash bucket, especially when calling btr_search_x_lock(index) and btr_search_x_unlock(index) in the same function.

It is better to cache the result of btr_get_search_latch() in a local variable and to pass that as parameter to certain functions.