net/sunrpc: return 0 on attempt to write to "transports"

You can't write to this file because the permissions are 0444.  But
it sort of looked like you could do a write and it would result in
a read.  Then it looked like proc_sys_call_handler() just ignored
it.  Which is confusing.  It's more clear if the "write" just
returns zero.

Also, the "lenp" pointer is never NULL so that check can be removed.

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