zram: Fix __zram_bvec_{read,write}() locking order

Mikhail reported a lockdep spat detailing how __zram_bvec_read() and
__zram_bvec_write() use zstrm->lock and zspage->lock in opposite order.

Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>