nfscl: Add support for a NFSv4 AppendWrite RPC

For IO_APPEND VOP_WRITE()s, the code first does a
Getattr RPC to acquire the file's size, before it
can do the Write RPC.

Although NFS does not have an append write operation,
an NFSv4 compound can use a Verify operation to check
that the client's notion of the file's size is
correct, followed by the Write operation.

This patch modifies nfscl_wcc_data() to optionally
acquire the file's size, for use with an AppendWrite.
Although the "stuff" arguments are always NULL
(these were used for the Mac OSX port and should be
cleared out someday), make the argument to
nfscl_wcc_data() explicitly NULL for clarity.

This patch does not cause any semantics change until
the AppendWrite is added in a future commit.

(cherry picked from commit 21de450aa1a7ca226908e7421b93ca979ff42db7)