MDEV-27058 fixup: Relax a debug assertion

buf_page_get_low(): Assert that the block not be read-fixed.
It may be write-fixed while we only hold a shared latch on the page.
Page writes are protected by U latches, which are compatible with S.
In all other places where we assert that the block not be IO-fixed,
we are holding U or X latch, which does prevent concurrent file I/O.

storage/innobase/buf/buf0buf.cc

@@ -2673,7 +2673,8 @@ buf_page_get_low(
 		after buf_zip_decompress() in this function. */
 		block->page.lock.s_lock();
 		state = block->page.state();
-		ut_ad(state < buf_page_t::READ_FIX);
+		ut_ad(state < buf_page_t::READ_FIX
+		      || state >= buf_page_t::WRITE_FIX);
 		const page_id_t id{block->page.id()};
 		block->page.lock.s_unlock();