drbd: fix request leak introduced by locking/atomic, kref: Kill kref_sub()

drbd: fix request leak introduced by locking/atomic, kref: Kill kref_sub()

commit a00ebd1cf12c378a1d4f7a1d6daf1d76c1eaad82 upstream.

When killing kref_sub(), the unconditional additional kref_get()
was not properly paired with the necessary kref_put(), causing
a leak of struct drbd_requests (~ 224 Bytes) per submitted bio,
and breaking DRBD in general, as the destructor of those "drbd_requests"
does more than just the mempoll_free().

Fixes: bdfafc4ffdd2 ("locking/atomic, kref: Kill kref_sub()")
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>