selftests/bpf/test_maps: exit child process without error in ENOMEM case

test_maps contains a series of stress tests, and previously it will break the
rest tests when it failed to alloc memory.
-----------------------
Failed to create hashmap key=8 value=262144 'Cannot allocate memory'
Failed to create hashmap key=16 value=262144 'Cannot allocate memory'
Failed to create hashmap key=8 value=262144 'Cannot allocate memory'
Failed to create hashmap key=8 value=262144 'Cannot allocate memory'
test_maps: test_maps.c:955: run_parallel: Assertion `status == 0' failed.
Aborted
not ok 1..3 selftests:  test_maps [FAIL]
-----------------------
after this patch, the rest tests will be continue when it occurs an ENOMEM failure

CC: Alexei Starovoitov <alexei.starovoitov@gmail.com>
CC: Philip Li <philip.li@intel.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Li Zhijian <zhijianx.li@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_maps.c

@@ -126,6 +126,8 @@ static void test_hashmap_sizes(int task, void *data)
 			fd = bpf_create_map(BPF_MAP_TYPE_HASH, i, j,
 					    2, map_flags);
 			if (fd < 0) {
+				if (errno == ENOMEM)
+					return;
 				printf("Failed to create hashmap key=%d value=%d '%s'\n",
 				       i, j, strerror(errno));
 				exit(1);