MFC 267328: change the netmap mbuf destructor so the same code works also on FreeBSD 9. For head and 10 this change has no effect, but on stable/9 it would cause panics when using emulated netmap on top of a standard device driver.
MFC 267328:
change the netmap mbuf destructor so the same code works also on FreeBSD 9.
For head and 10 this change has no effect, but on stable/9 it would cause
panics when using emulated netmap on top of a standard device driver.

MFC after:	3 days

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