common: open files in ro mode for extent and hole count helpers

The helper functions _count_extents() and _count_holes() open their
input file in RW mode to call fiemap, however opening it in RO mode
is enough.  By opening them in RW mode it also makes it not possible
to use them against files residing in btrfs readonly snapshots for
example.

So just open the files in RO mode in these functions.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>