samples, bpf: Suppress compiler warning

While cross compiling on ARM32 , the casting from pointer to __u64 will
cause warnings:

samples/bpf/task_fd_query_user.c: In function 'main':
samples/bpf/task_fd_query_user.c:399:23: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
399 | uprobe_file_offset = (__u64)main - (__u64)&__executable_start;
| ^
samples/bpf/task_fd_query_user.c:399:37: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
399 | uprobe_file_offset = (__u64)main - (__u64)&__executable_start;

Workaround this by using "unsigned long" to adapt to different ARCHs.
Signed-off-by: Hailong Liu <liu.hailong6@zte.com.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210511140429.89426-1-liuhailongg6@163.com

samples/bpf/task_fd_query_user.c

@@ -396,7 +396,7 @@ int main(int argc, char **argv)
 	 * on different systems with different compilers. The right way is
 	 * to parse the ELF file. We took a shortcut here.
 	 */
-	uprobe_file_offset = (__u64)main - (__u64)&__executable_start;
+	uprobe_file_offset = (unsigned long)main - (unsigned long)&__executable_start;
 	CHECK_AND_RET(test_nondebug_fs_probe("uprobe", (char *)argv[0],
 					     uprobe_file_offset, 0x0, false,
 					     BPF_FD_TYPE_UPROBE,