svcrpc: delay minimum-rpc-size check till later

svcrpc: delay minimum-rpc-size check till later

Soon we want to support multiple fragments, in which case it may be
legal for a single fragment to be smaller than 8 bytes, so we'll want to
delay this check till we've reached the last fragment.

Also fix an outdated comment.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>