MDEV-25998 fixup: Avoid a hang

btr_scrub_start_space(): Avoid an unnecessary tablespace lookup
and related acquisition of fil_system->mutex. In MariaDB Server 10.3
we would get deadlocks between that mutex and a crypt_data mutex.

The fix was developed by Thirunarayanan Balathandayuthapani.

storage/innobase/btr/btr0scrub.cc

 // Copyright (c) 2014, Google Inc.
-// Copyright (c) 2017, MariaDB Corporation.
+// Copyright (c) 2017, 2021, MariaDB Corporation.
 
 /**************************************************//**
 @file btr/btr0scrub.cc
@@ -835,20 +835,12 @@ btr_scrub_page(
 
 /**************************************************************//**
 Start iterating a space */
-UNIV_INTERN
-bool
-btr_scrub_start_space(
-/*===================*/
-	ulint space,             /*!< in: space */
-	btr_scrub_t* scrub_data) /*!< in/out: scrub data */
+bool btr_scrub_start_space(const fil_space_t &space, btr_scrub_t *scrub_data)
 {
-	bool found;
-	scrub_data->space = space;
+	scrub_data->space = space.id;
 	scrub_data->current_table = NULL;
 	scrub_data->current_index = NULL;
-	const page_size_t page_size = fil_space_get_page_size(space, &found);
-
-	scrub_data->compressed = page_size.is_compressed();
+	scrub_data->compressed = FSP_FLAGS_GET_ZIP_SSIZE(space.flags) != 0;
 	scrub_data->scrubbing = check_scrub_setting(scrub_data);
 	return scrub_data->scrubbing;
 }

storage/innobase/fil/fil0crypt.cc

@@ -1200,7 +1200,7 @@ fil_crypt_space_needs_rotation(
 			key_state->rotate_key_age);
 
 		crypt_data->rotate_state.scrubbing.is_active =
-			btr_scrub_start_space(space->id, &state->scrub_data);
+			btr_scrub_start_space(*space, &state->scrub_data);
 
 		time_t diff = time(0) - crypt_data->rotate_state.scrubbing.
 			last_scrub_completed;

storage/innobase/include/btr0scrub.h

@@ -141,12 +141,7 @@ btr_scrub_skip_page(
 /****************************************************************
 Start iterating a space
 * @return true if scrubbing is turned on */
-UNIV_INTERN
-bool
-btr_scrub_start_space(
-/*===================*/
-	ulint space,              /*!< in: space */
-	btr_scrub_t* scrub_data); /*!< in/out: scrub data */
+bool btr_scrub_start_space(const fil_space_t &space, btr_scrub_t *scrub_data);
 
 /** Complete iterating a space.
 @param[in,out]	scrub_data	 scrub data */