net/sunrpc: fix useless comparison in proc_do_xprt()

In the original code, the "if (*lenp < 0)" check didn't work because
"*lenp" is unsigned.  Fortunately, the memory_read_from_buffer() call
will never fail in this context so it doesn't affect runtime.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>