Fix a deadlock in the initial patch. lock_validate() must not hold the
lock system mutex while s-latching a block, because some functions,
such as lock_rec_convert_impl_to_expl(), may be already holding an x-latch
on the block that lock_validate() is interested in while attempting to
acquire the lock system mutex.

This deadlock was not caught by UNIV_SYNC_DEBUG because of
buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK).