selftests/bpf: Fix build error with llvm 19

llvm 19 fails to compile arena self test:
CLNG-BPF [test_progs] verifier_arena_large.bpf.o
progs/verifier_arena_large.c:90:24: error: unsupported signed division, please convert to unsigned div/mod.
   90 |                 pg_idx = (pg - base) / PAGE_SIZE;

Though llvm <= 18 and llvm >= 20 don't have this issue,
fix the test to avoid the build error.

Reported-by: Jiri Olsa <olsajiri@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>