Calling discard_cache() is likely to result in a call to
free_memblock(), which will call valgrind_make_blocks_noaccess()
before returning. This causes valgrind to report an invalid read on
the next iteration through the loop in alloc_memblock().
Fix by explicitly calling valgrind_make_blocks_defined() after
discard_cache() returns. Also call valgrind_make_blocks_noaccess()
before calling discard_cache(), to guard against free list corruption
while executing cache discarders.
Signed-off-by: Michael Brown <mcb30@ipxe.org>