ostree-repo: Make the lock with a long-lasting FD

ostree-repo: Make the lock with a long-lasting FD

glnx_make_lock_file requires that the dfd passed in survives the
lifetime of the lock. Since dfd_iter.fd gets cleaned up after the
function returns, this isn't the case. dfd_iter.fd should be equivalent
to tmpdir_dfd, since we iter on ".", and that survives past the
function, so just use that instead.

Closes: #591
Approved by: cgwalters