nfs: Fix misparsing of nfsv4 fs_locations attribute

The code incorrectly assumes here that the server name (or ip address)
is null-terminated.  This can cause referrals to fail in some cases.

Also support ipv6 addresses.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>