qed: Add mpa buffer descriptors for storing and processing mpa fpdus

The mpa buff is a descriptor for iwarp ll2 buffers that contains
additional information required for aligining fpdu's.
In some cases, an additional packet will arrive which will complete
the alignment of a fpdu, but we won't be able to post the fpdu due to
insufficient place on the tx ring. In this case we can't loose the data
and require storing it for later. Processing is therefore done
in two places, during rx completion, where we initialize a mpa buffer
descriptor and add it to the pending list, and during tx-completion, since
we free up an entry in the tx chain we can process any pending mpa packets.
The mpa buff descriptors are pre-allocated since we have to ensure that
we won't reach a state where we can't store an incoming unaligned packet.
All packets received on the ll2 MUST be processed by the driver at some
stage. Since they are preallocated, we hold a free list.

Signed-off-by: Michal Kalderon <[email protected]>
Signed-off-by: Ariel Elior <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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