Btrfs-progs: allow compression property gets for read-only subvolumes

Btrfs-progs: allow compression property gets for read-only subvolumes

Because the function open_file_or_dir() always opened the input file in
read/write mode (O_RDWR), we were not able to due a compression property
get against a file living in a read-only subvolume/snapshot.
Fix this by opening the file with O_RDONLY mode if we're doing a property
get.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>