selftests/bpf: Fix conflicts with built-in functions in bench_local_storage_create

The fork function in gcc is considered a built in function due to
being used by libgcov when building with gnu extensions.

Rename fork to sched_process_fork to prevent this conflict.

See details:
https://github.com/gcc-mirror/gcc/commit/d1c38823924506d389ca58d02926ace21bdf82fa
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82457

Fixes the following error:

In file included from progs/bench_local_storage_create.c:6:
progs/bench_local_storage_create.c:43:14: error: conflicting types for
built-in function 'fork'; expected 'int(void)'
[-Werror=builtin-declaration-mismatch]
   43 | int BPF_PROG(fork, struct task_struct *parent, struct
task_struct *child)
      |              ^~~~

Fixes: cbe9d93d58 ("selftests/bpf: Add bench for task storage creation")
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230331075848.1642814-1-james.hilliard1@gmail.com

tools/testing/selftests/bpf/benchs/bench_local_storage_create.c

@@ -95,7 +95,7 @@ static void setup(void)
 			exit(1);
 		}
 	} else {
-		if (!bpf_program__attach(skel->progs.fork)) {
+		if (!bpf_program__attach(skel->progs.sched_process_fork)) {
 			fprintf(stderr, "Error attaching bpf program\n");
 			exit(1);
 		}

tools/testing/selftests/bpf/progs/bench_local_storage_create.c

@@ -40,7 +40,7 @@ int BPF_PROG(kmalloc, unsigned long call_site, const void *ptr,
 }
 
 SEC("tp_btf/sched_process_fork")
-int BPF_PROG(fork, struct task_struct *parent, struct task_struct *child)
+int BPF_PROG(sched_process_fork, struct task_struct *parent, struct task_struct *child)
 {
 	struct storage *stg;