Merge branch 'bpf: refine retval for bpf_get_task_stack helper'

Dave Marchevsky says:

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Similarly to the bpf_get_stack helper, bpf_get_task_stack's return value
can be more tightly bound by the verifier - it's the number of bytes
written to a user-supplied buffer, or a negative error value. Currently
the verifier believes bpf_task_get_stack's retval bounds to be unknown,
requiring extraneous bounds checking to remedy.

Adding it to do_refine_retval_range fixes the issue, as evidenced by
new selftests which fail to load if retval bounds are not refined.

v2: Addressed comment nit in patch 3
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>