bpf: Fix tests for local_storage

The {inode,sk}_storage_result checking if the correct value was retrieved
was being clobbered unconditionally by the return value of the
bpf_{inode,sk}_storage_delete call.

Also, consistently use the newly added BPF_LOCAL_STORAGE_GET_F_CREATE
flag.

Fixes: cd324d7a

 ("bpf: Add selftests for local_storage")
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201106103747.2780972-7-kpsingh@chromium.org