pass kernel pointers to the sysctl ->proc_handler method v3

[Christoph Hellwig]

Hi all,

this series changes the sysctl ->proc_handler methods to take kernel
pointers.  This simplifies some of the pointer handling in the methods
(which could probably be further simplified now), and gets rid of the
set_fs address space overrides used by bpf.

Changes since v2:
 - free the buffer modified by BPF
 - move pid_max and friends to pid.h

Changes since v1:
 - drop a patch merged by Greg
 - don't copy data out on a write
 - fix buffer allocation in bpf