block-map: add __GFP_ZERO flag for alloc_page in function bio_copy_kern

commit cc8f7fe1 upstream.

Add __GFP_ZERO flag for alloc_page in function bio_copy_kern to initialize
the buffer of a bio.

Signed-off-by: Haimin Zhang <tcs.kernel@gmail.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220216084038.15635-1-tcs.kernel@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
[nobelbarakat: Backported to 5.10: Manually added flag] 
Signed-off-by: Nobel Barakat <nobelbarakat@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>