aoe: update cap on outstanding commands based on config query response

The ATA over Ethernet config query response contains a "buffer count"
field reflecting the AoE target's capacity to buffer incoming AoE
commands.

By taking the current value of this field into accound, we increase
performance throughput or avoid network congestion, when the value
has increased or decreased, respectively.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>