MFC: r237219

MFC: r237219

In tty_makedev() the following construction:

dev = make_dev_cred();
dev->si_drv1 = tp;

leaves a small window where the newly created device may be opened
and si_drv1 is NULL.

As this is a vary rare situation, using a lock to close the window
seems overkill. Instead just wait for the assignment of si_drv1.

git-svn-id: svn://svn.freebsd.org/base/stable/8@237780 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f