bpf: improve stacksafe state comparison

"if (old->allocated_stack > cur->allocated_stack)" check is too conservative.
In some cases explored stack could have allocated more space,
but that stack space was not live.
The test case improves from 19 to 15 processed insns
and improvement on real programs is significant as well:

                       before    after
bpf_lb-DLB_L3.o        1940      1831
bpf_lb-DLB_L4.o        3089      3029
bpf_lb-DUNKNOWN.o      1065      1064
bpf_lxc-DDROP_ALL.o    28052     26309
bpf_lxc-DUNKNOWN.o     35487     33517
bpf_netdev.o           10864     9713
bpf_overlay.o          6643      6184
bpf_lcx_jit.o          38437     37335

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Edward Cree <ecree@solarflare.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>