btrfs: rename __get_extent_map() and pass btrfs_inode

The double underscore naming scheme does not apply here, there's only
only get_extent_map(). As the definition is changed also pass the struct
btrfs_inode.

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