[BZ #22830] malloc_stats: restore cancellation for stderr correctly.

malloc_stats means to disable cancellation for writes to stderr while
it runs, but it restores stderr->_flags2 with |= instead of =, so what
it actually does is disable cancellation on stderr permanently.

	[BZ #22830]
	* malloc/malloc.c (__malloc_stats): Restore stderr->_flags2
        correctly.
        * malloc/tst-malloc-stats-cancellation.c: New test case.
        * malloc/Makefile: Add new test case.