Revert "md: Don't clear MD_CLOSING when the raid is about to stop"

This reverts commit bb567cb5 which is
commit 9674f54e upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I4c121821592a6813e9125c7946fc43240c1304e1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>