net: sunrpc: Fix off-by-one issues in 'rpc_ntop6'

Fix off-by-one issues in 'rpc_ntop6':
 - 'snprintf' returns the number of characters which would have been
   written if enough space had been available, excluding the terminating
   null byte. Thus, a return value of 'sizeof(scopebuf)' means that the
   last character was dropped.
 - 'strcat' adds a terminating null byte to the string, thus if len ==
   buflen, the null byte is written past the end of the buffer.

Signed-off-by: Fedor Tokarev <ftokarev@gmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>