aoe: add module parameter for users who need more outstanding I/O

An AoE target provides an estimate of the number of outstanding commands that
the AoE initiator can send before getting a response.  The aoe_maxout
parameter provides a way to set an even lower limit.  It will not allow a user
to use more outstanding commands than the target permits.  If a user discovers
a problem with a large setting, this parameter provides a way for us to work
with them to debug the problem.  We expect to improve the dynamic window
sizing algorithm and drop this parameter.  For the time being, it is a
debugging aid.

Signed-off-by: Ed L. Cashin <ecashin@coraid.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>