MDEV-14952 Avoid repeated calls to btr_get_search_latch()

btr_cur_search_to_nth_level(), row_sel(): Do not bother to yield to
waiting exclusive lock requests on the adaptive hash index latch.
When the btr_search_latch was split into an array of latches
in MySQL 5.7.8 as part of the Oracle Bug#20985298 fix, the "caching"
of the latch across storage engine API calls was removed. Thus,
X-lock requests should have a good chance of becoming served, and
starvation should not be possible.

btr_search_guess_on_hash(): Clean up a debug assertion.

storage/innobase/btr/btr0cur.cc

@@ -3,7 +3,7 @@
 Copyright (c) 1994, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2015, 2017, MariaDB Corporation.
+Copyright (c) 2015, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -1129,8 +1129,6 @@ btr_cur_search_to_nth_level(
 	    && btr_search_enabled
 	    && !modify_external
 	    && !(tuple->info_bits & REC_INFO_MIN_REC_FLAG)
-	    && rw_lock_get_writer(btr_get_search_latch(index))
-	    == RW_LOCK_NOT_LOCKED
 	    && btr_search_guess_on_hash(index, info, tuple, mode,
 					latch_mode, cursor,
 					has_search_latch, mtr)) {

storage/innobase/btr/btr0sea.cc

@@ -1021,8 +1021,7 @@ btr_search_guess_on_hash(
 		}
 	}
 
-	ut_ad(rw_lock_get_writer(btr_get_search_latch(index)) != RW_LOCK_X);
-	ut_ad(rw_lock_get_reader_count(btr_get_search_latch(index)) > 0);
+	ut_ad(rw_lock_own(btr_get_search_latch(index), RW_LOCK_S));
 
 	rec = (rec_t*) ha_search_and_get_data(
 			btr_get_search_table(index), fold);