MDEV-23399 fixup for MDEV-10814

buf_madvise_do_dump(): Fix a mutex release that was broken in commit 7cffb5f. This function is not covered by any tests. Its only purpose is to be called from a debugger so that buffers that would normally be excluded from core dumps ever since commit b600f30 can be included.

Author: dr-m

storage/innobase/buf/buf0buf.cc

@@ -1021,7 +1021,7 @@ buf_madvise_do_dump()
 ret+= madvise(chunk->mem, chunk->mem_size(), MADV_DODUMP);
 }
 
-mysql_mutex_lock(&buf_pool.mutex);
+mysql_mutex_unlock(&buf_pool.mutex);
 return ret;
 }
 #endif