btrfs: assert proper locks are held at tree_insert_offset()

There are multiple code paths leading to tree_insert_offset(), and each
path takes the necessary locks before tree_insert_offset() is called,
since they do other things that require those locks to be held. This makes
it easy to miss the locking somewhere, so make tree_insert_offset() assert
that the required locks are being held by the calling task.

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