CFR: FireWire: Don't allow a tlabel to reference an xfer after free

[Will Andrews]

Diff: http://people.freebsd.org/~will/patches/fix-fwmem-use-after-free.diff

>From the commit log:

	FireWire: Don't allow a tlabel to reference an xfer after free.
	
	sys/dev/firewire/firewire.c:
	- fw_xfer_unload(): Since we are about to free this xfer, call
	  fw_tl_free() to remove the xfer from its tlabel's list, if
	  it has a tlabel.
	- In every occasion when a xfer is removed from a tlabel's list,
	  reset xfer->tl to -1 while holding fc->tlabel_lock, so that the
	  xfer isn't mis-identified as belonging to a tlabel.


Thanks,
--Will.