Fix some problems that were introduced in revision 1.97. Instead

Fix some problems that were introduced in revision 1.97. Instead
of returning an error code to the caller, NFS server op routines
must themselves build an error reply and return 0 to the caller.

This is achieved by replacing the erroneous return statements with
code that jumps forward to the op function's reply code. We need
to be careful to ensure that the 'struct mount' pointer is NULL
though, so that the final vn_finished_write() call becomes a no-op.

Reviewed by:	mckusick, dillon