Syzbot reported a kernel warning due to a range invariant violation in
the BPF verifier. The issue occurs when tnum_overlap() fails to detect
that two tnums don't have any overlapping bits.

The problematic BPF program:
   0: call bpf_get_prandom_u32
   1: r6 = r0
   2: r6 &= 0xFFFFFFFFFFFFFFF0
   3: r7 = r0
   4: r7 &= 0x07
   5: r7 -= 0xFF
   6: if r6 == r7 goto <exit>

After instruction 5, R7 has the range:
   R7: u64=[0xffffffffffffff01, 0xffffffffffffff08] var_off=(0xffffffffffffff00; 0xf)

R6 and R7 don't overlap since they have no agreeing bits. However,
is_branch_taken() fails to recognize this, causing the verifier to
refine register bounds and trigger range bounds violation:

   6: if r6 == r7 goto <exit>
   true_reg1: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)
   true_reg2: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0)

The root cause is that tnum_overlap() doesn't properly handle the case
where the masks have no overlapping bits.

Fix this by adding an early check for zero mask intersection in tnum_overlap().

Reported-by: [email protected]
Fixes: f41345f ("bpf: Use tnums for JEQ/JNE is_branch_taken logic")
Signed-off-by: KaFai Wan <[email protected]>
Reported-by: [email protected]