bpf: Fix the irq and nmi check in bpf_sk_storage for tracing usage

The intention of the current check is to avoid using bpf_sk_storage
in irq and nmi.  Jakub pointed out that the current check cannot
do that.  For example, in_serving_softirq() returns true
if the softirq handling is interrupted by hard irq.

Fixes: 8e4597c6

 ("bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW_TP")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201116200113.2868539-1-kafai@fb.com