NFS: fix nfs_parse_ip_address() corner case

Bruce observed that nfs_parse_ip_address() will successfully parse an
IPv6 address that looks like this:

  "::1%"

A scope delimiter is present, but there is no scope ID following it.
This is harmless, as it would simply set the scope ID to zero.  However,
in some cases we would like to flag this as an improperly formed
address.

We are now also careful to reject addresses where garbage follows the
address (up to the length of the string), instead of ignoring the
non-address characters; and where the scope ID is nonsense (not a valid
device name, but also not numeric).  Before, both of these cases would
result in a harmless zero scope ID.

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