selftests/bpf: Add test for map_ptr arithmetic

Change selftest map_ptr_kern.c with disabling inlining for
one of subtests, which will fail the test without previous
verifier change. Also added to verifier test for both
"map_ptr += scalar" and "scalar += map_ptr" arithmetic.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200908175703.2463721-1-yhs@fb.com