NFS: Allow the mount option retrans=0

We should allow retrans=0 as just meaning that every timeout is a major
timeout, and that there is no increment in the timeout value.

For instance, this means that we would allow TCP users to specify a
flat timeout value of 60s, by specifying "timeo=600,retrans=0" in their
mount option string.

Siged-off-by: Trond Myklebust <trond.myklebust@primarydata.com>