btrfs: simplify arguments to tree_insert_offset()

For the in-memory component of space caching (free space cache and free
space tree), three of the arguments passed to tree_insert_offset() can
always be taken from the new free space entry that we are about to add.

So simplify tree_insert_offset() to take the new entry instead of the
'offset', 'node' and 'bitmap' arguments. This will also allow to make
further changes simpler.

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