bpf: Initialize used but uninit variable in propagate_liveness()

With input changed == NULL, a local variable is used for "changed".
Initialize tmp properly, so that it can be used in the following:
   *changed |= err > 0;

Otherwise, UBSAN will complain:

UBSAN: invalid-load in kernel/bpf/verifier.c:18924:4
load of value <some random value> is not a valid value for type '_Bool'

Fixes: dfb2d4c6 ("bpf: set 'changed' status if propagate_liveness() did any updates")
Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250612221100.2153401-1-song@kernel.org


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