btrfs: warn if extent buffer mapping crosses a page boundary in csum_tree_block

Since commit d2e174d5

 ("btrfs: document extent mapping assumptions in
checksum") we have a comment in place why map_private_extent_buffer()
can't return 1 in the csum_tree_block() case.

Make this a bit more explicit and WARN_ON() in case this this assumption
breaks.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>