MFC r359159:

cxgbe(4): Split sge_nm_rxq into three cachelines.

This reduces the lines bouncing around between the driver rx ithread and
the netmap rxsync thread. There is no net change in the size of the
struct (it continues to waste a lot of space).

This kind of split was originally proposed in D17869 by Marc De La
Gueronniere @ Verisign, Inc.

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