nfscl: Clean up the code by removing unused arguments

The "void *stuff" (also called fstuff and dstuff) argument
was used by the Mac OSX port. For FreeBSD, this argument
is always NULL, so remove it to clean up the code.

This commit gets rid of "stuff" for assorted functions
defined in nfs_clrpcops.c and called in nfs_clvnops.c and
nfs_clvfsops.c. Future commits will do the same for other functions.

This commit should not result in a semantics change.