btrfs: reduce the variable size to fit nr_pages

Patch "btrfs: reduce compressed_bio member's types" reduced the
@nr_pages size to unsigned int, its cascading effects are updated here.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>