btrfs: replace memset with memzero_page in data checksum verification

The original code resets the page to 0x1 for not apparent reason, it's
been like that since the initial 2007 code added in commit 07157aac
("Btrfs: Add file data csums back in via hooks in the extent map code").

It could mean that a failed buffer can be detected from the data but
that's just a guess and any value is good.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>