btrfs: add btrfs prefix to free space tree exported functions

btrfs: add btrfs prefix to free space tree exported functions

A few of the free space tree exported functions have a 'btrfs_' prefix in
their name, but most don't. Not only is this inconsistent, the preferred
style is to have such a prefix, to avoid potential collisions in the
future with other kernel code and offer a somewhat better readibility by
making it obvious in calls sites that we are calling btrfs specific code.

So add the 'btrfs_' prefix to all free space tree functions that are
missing it.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>