MDEV-24142: Avoid block_lock alignment loss on 64-bit systems

sux_lock::recursive: Move right after the 32-bit sux_lock::lock.
This will reduce sizeof(block_lock) from 24 to 16 bytes on
64-bit systems with CMAKE_BUILD_TYPE=RelWithDebInfo. This may be
significant, because there will be one buf_block_t::lock for each
buffer pool page descriptor.

We still have some potential for savings, with sizeof(buf_page_t)==112
and sizeof(buf_block_t)==184 on a GNU/Linux AMD64 system.

Note: On GNU/Linux AMD64, sizeof(index_lock) remains 32 bytes
(16 with PLUGIN_PERFSCHEMA=NO) even tough it would fit in 24 bytes.
This is because sizeof(srw_lock) includes 4 bytes of padding
(to 16 bytes) that index_lock_t::recursive cannot reuse. So,
in total 4+4 bytes will be lost to padding. This is rather
insignificant compared to sizeof(dict_index_t)==400.

storage/innobase/include/srw_lock.h

@@ -114,8 +114,8 @@ typedef srw_lock_low srw_lock;
 /** Slim reader-writer lock with PERFORMANCE_SCHEMA instrumentation */
 class srw_lock
 {
-  srw_lock_low lock;
   PSI_rwlock *pfs_psi;
+  srw_lock_low lock;
 
   template<bool support_u_lock>
   ATTRIBUTE_NOINLINE void psi_rd_lock(const char *file, unsigned line);
@@ -126,8 +126,8 @@ class srw_lock
 public:
   void init(mysql_pfs_key_t key)
   {
-    lock.init();
     pfs_psi= PSI_RWLOCK_CALL(init_rwlock)(key, this);
+    lock.init();
   }
   void destroy()
   {

storage/innobase/include/sux_lock.h

@@ -33,6 +33,8 @@ class sux_lock final
 {
   /** The underlying non-recursive lock */
   srw lock;
+  /** Numbers of U and X locks. Protected by lock. */
+  uint32_t recursive;
   /** The owner of the U or X lock (0 if none); protected by lock */
   std::atomic<os_thread_id_t> writer;
   /** Special writer!=0 value to indicate that the lock is non-recursive
@@ -42,8 +44,6 @@ class sux_lock final
 #else
 # define FOR_IO ((os_thread_id_t) ~0UL) /* it could be a pointer */
 #endif
-  /** Numbers of U and X locks. Protected by lock. */
-  uint32_t recursive;
 #ifdef UNIV_DEBUG
   /** Protects readers */
   mutable srw_mutex readers_lock;