btrfs: use pgoff_t for page index variables

btrfs: use pgoff_t for page index variables

Any conversion of offsets in the logical or the physical mapping space
of the pages is done by a shift and the target type should be pgoff_t
(type of struct page::index). Fix the locations where it's still
unsigned long.

Signed-off-by: David Sterba <dsterba@suse.com>