btrfs: tree-checker: extract the shared key check for free space entries

btrfs: tree-checker: extract the shared key check for free space entries

Currently both check_free_space_extent() and check_free_space_bitmap()
share a very common validation on the keys.

Extract them into a helper, check_free_space_common_key(), and
change the output string ("extent" or "bitmap") depending on the key type.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>