mount.nfs: Mount should really return from errno test

We should only try next address family if we meet ECONNREFUSED or
EHOSTUNREACH for v4 or ECONNREFUSED or EOPNOTSUPP or EHOSTUNREACH for v3v2.
Before, only a break in swich can not make the program out of for loop.

Signed-off-by: Yang Bai <hamo.by@gmail.com>
Signed-off-by: Steve Dickson <steved@redhat.com>