nfs4: do strict utf8 check

Avoid surprises in the file system.

Java uses internally UTF-16 and, as a result, encode accepts non valid
surrogate bytes as well. For NFs such sequences are invalid.

See: http://www.oracle.com/technetwork/articles/javase/supplementary-142654.html#Modified_UTF-8

Acked-by: Gerd Behrmann
Target: master