nfscl: Do not flush when a write delegation is held

When a NFSv4 byte range write lock is unlocked, all
data modifications need to be flushed to the server
to satisfy the coherency requirements for byte range
locking. However, if a write delegation for the
file is held by the client, flushing is not required,
since no other NFSv4 client can have the file NFSv4
Opened.

Found by inspection as suggested by a similar change
that was done to the Linux NFSv4 client.