clnt_vc.c: Replace msleep() with pause() to avoid assert panic

An msleep() in clnt_vc.c used a global "fake_wchan" wchan argument
along with the mutex in a CLIENT structure. As such, it was
possible to use different mutexes for the same wchan and
cause a panic assert. Since this is in a rarely executed code
path, the assert panic was only recently observed.

Since "fake_wchan" never gets a wakeup, this msleep() can
be replaced with a pause() to avoid the panic assert,
which is what this patch does.

Reviewed by: kib, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D36977