ixl(4)/ixlv(4): Revert m_collapse() in ixl_xmit() to m_defrag().

The m_collapse() call would fail when transmitting medium-sized packets
when the interface mtu was set to 9000, so revert back to m_defrag(),
which does not fail.

Differential Revision: https://reviews.freebsd.org/D5207
Tested by: jeffrey.e.pieper@intel.com
Sponsored by: Intel Corporation