md/raid10: fix memory leak when reshaping a RAID10.

commit ce0b0a46955d1bb389684a2605dbcaa990ba0154 upstream.

raid10 reshape clears unwanted bits from a bio->bi_flags using
a method which, while clumsy, worked until 3.10 when BIO_OWNS_VEC
was added.
Since then it clears that bit but shouldn't.  This results in a
memory leak.

So change to used the approved method of clearing unwanted bits.

As this causes a memory leak which can consume all of memory
the fix is suitable for -stable.

Fixes: a38352e0
Reported-by: mdraid.pkoch@dfgh.net (Peter Koch)
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>