m_getjcl and packet cache

	Hi,

I've posted on -net but in case people will have some feedback on this before commit.
There is two solutions, let every driver get a test for MCLBYTES and call m_getcl (and then add support for at least igb, em, ixgbe) 
or integrate the fast path in m_getjcl.

I've chosen to integrate the fast path in m_getjcl and patched the two case now redundant in the attached patch.

From freebsd-net_at_:

While doing some 10Gb benchmark i've found that m_getjcl does not benefit from the packet zone.
There is a ~ 80% increase in FPS when applying the following patch.

256B frame driver to driver / stable_8:
- 3 765 066 FPS
- 6 868 153 FPS with the patch applied.

Any advice ?

Fabien