[PATCH] loopback: #ifdef the TSO code

This patch #ifdefs the TSO code in the loopback driver.

Saves ~800 bytes of text on i386 and avoids a conditional in the fast path.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>