[PATCH] aoe 9/12: add note about the need for deadlock-free sk_buff allocation

There is a potential for deadlock when allocating a struct sk_buff for

data that needs to be written out to aoe storage.  If the data is

being written from a dirty page in order to free that page, and if

there are no other pages available, then deadlock may occur when a

free page is needed for the sk_buff allocation.  This situation has

not been observed, but it would be nice to eliminate any potential for

deadlock under memory pressure.

Because ATA over Ethernet is not fragmented by the kernel's IP code,

the destructore member of the struct sk_buff is available to the aoe

driver.  By using a mempool for allocating all but the first few

sk_buffs, and by registering a destructor, we should be able to

efficiently allocate sk_buffs without introducing any potential for

deadlock.