btrfs: assert tree lock is held when searching for free space entries

When searching for a free space entry by offset, at tree_search_offset(),
we are supposed to have the btrfs_free_space_ctl's 'tree_lock' held, so
assert that. We have multiple callers of tree_search_offset(), and all
currently hold the necessary lock before calling it.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>