[PATCH] nbd: fix struct request race condition

[PATCH] nbd: fix struct request race condition

Here's a patch to fix a race condition in nbd that was causing struct
request corruption (requests were being freed while still in use).  This
patch improves on the previous one, which admittedly was a bit dodgy, using
struct request's ref_count field (I should have listened to Jens in the
first place :).  This should fix all the corner cases related to struct
request usage/freeing in nbd.  My stress tests do a lot better with this
patch applied.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>