zram: use memcpy_from_bvec in zram_bvec_write

Use memcpy_from_bvec instead of open coding the logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20220303111905.321089-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>