selftests/bpf: Fix test for refinement of single-value tnum

This patch fixes the "bounds refinement with single-value tnum on umin"
verifier selftest. This selftest was introduced in commit e6ad477d1bf8
("selftests/bpf: Test refinement of single-value tnum") to cover the
logic from __update_reg64_bounds(), introduced in commit efc11a667878
("bpf: Improve bounds when tnum has a single possible value"). However,
the test still passes if that last commit is reverted.

The test is supposed to cover the case when the tnum and u64 range (or
cnum64 now) overlap in a single value. __update_reg64_bounds() detects
that case and refines the bounds to a known constant. However, the
constants for the test were poorly chosen and the bounds get refined to
a known constant even without __update_reg64_bounds(). The code is as
follows:

  0: call bpf_get_prandom_u32#7  ; R0=scalar()
  1: r0 |= 224                   ; R0=scalar(umin=umin32=224,var_off=(0xe0; 0xffffffffffffff1f))
  2: r0 &= 240                   ; R0=scalar(smin=umin=smin32=umin32=224,smax=umax=smax32=umax32=240,var_off=(0xe0; 0x10))
  3: if r0 == 0xf0 goto pc+2     ; R0=224

After instruction 3, we have u64=[0xe0; 0xef] and tnum=(0xe0; 0x10).
__reg_bound_offset() is able to deduce a new tnum from the u64,
tnum=(0xe0; 0x0f), which combined with the existing tnum gives us a
constant: 0xe0 or 224.

We can easily fix this by choosing different starting bounds. If we make
it u64=[0xe1; 0xf0], then __reg_bound_offset() doesn't have any impact.

Fixes: e6ad477d1bf8 ("selftests/bpf: Test refinement of single-value tnum")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/be2dc2c3d85120286e60b3029b3338fff339f942.1779121582.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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