bpf: Fix accesses to uninit stack slots

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix accesses to uninit stack slots

Privileged programs are supposed to be able to read uninitialized stack
memory (ever since 6715df8d5) but, before this patch, these accesses
were permitted inconsistently. In particular, accesses were permitted
above state->allocated_stack, but not below it. In other words, if the
stack was already "large enough", the access was permitted, but
otherwise the access was rejected instead of being allowed to "grow the
stack". This undesired rejection was happening in two places:

• in check_stack_slot_within_bounds()
• in check_stack_range_initialized()
  This patch arranges for these accesses to be permitted. A bunch of tests
  that were relying on the old rejection had to change; all of them were
  changed to add also run unprivileged, in which case the old behavior
  persists. One tests couldn't be updated - global_func16 - because it
  can't run unprivileged for other reasons.

This patch also fixes the tracking of the stack size for variable-offset
reads. This second fix is bundled in the same commit as the first one
because they're inter-related. Before this patch, writes to the stack
using registers containing a variable offset (as opposed to registers
with fixed, known values) were not properly contributing to the
function's needed stack size. As a result, it was possible for a program
to verify, but then to attempt to read out-of-bounds data at runtime
because a too small s…