uml: send pointers instead of structures to I/O thread

Instead of writing entire structures between UML and the I/O thread, we send
pointers.  This cuts down on the amount of data being copied and possibly
allows more requests to be pending between the two.

This requires that the requests be kmalloced and freed instead of living on
the stack.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>