slub: make ->offset unsigned int

slub: make ->offset unsigned int

->offset is free pointer offset from the start of the object, can't be
negative.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexey Dobriyan <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>