btrfs: move up backref sharedness cache store and lookup functions

Move the static functions to lookup and store sharedness check of an
extent buffer to a location above find_all_parents(), because in the
next patch the lookup function will be used by find_all_parents().
The store function is also moved just because it's the counter part
to the lookup function and it's best to have their definitions close
together.

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