[2/3] IRDA stack should call irda_get_skb_cb (tx_extra implementation)

A rewrite of the original patch by Samuel Ortiz, for the tx_extra 
implementation. Does away with irda_alloc_skb which is irrelevant for 
this implementation.

Based on a patch by Samuel Ortiz [off-list ref]

Signed-off-by: Alex Villacís Lasso <redacted>

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perl -e '$x=2.4;print sprintf("%.0f + %.0f = %.0f\n",$x,$x,$x+$x);'