bpf: allow setting mount device for bpffs

We noticed our tc ebpf tools can't start after we upgrade our in-house
kernel version from 4.19 to 5.10. That is because of the behaviour change
in bpffs caused by commit
d2935de ("vfs: Convert bpf to use the new mount API").

In our tc ebpf tools, we do strict environment check. If the enrioment is
not match, we won't allow to start the ebpf progs. One of the check is
whether bpffs is properly mounted. The mount information of bpffs in
kernel-4.19 and kernel-5.10 are as follows,

- kenrel 4.19
$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
bpffs on /sys/fs/bpf type bpf (rw,relatime)

- kernel 5.10
$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
none on /sys/fs/bpf type bpf (rw,relatime)

The device name in kernel-5.10 is displayed as none instead of bpffs,
then our environment check fails. Currently we modify the tools to adopt to
the kernel behaviour change, but I think we'd better change the kernel code
to keep the behavior consistent.

After this change, the mount information will be displayed the same with
the behavior in kernel-4.19, for example,

$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
bpffs on /sys/fs/bpf type bpf (rw,relatime)

Fixes: d2935de ("vfs: Convert bpf to use the new mount API")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: David Howells <dhowells@redhat.com>

kernel/bpf/inode.c

@@ -648,12 +648,26 @@ static int bpf_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	int opt;
 
 	opt = fs_parse(fc, bpf_fs_parameters, param, &result);
-	if (opt < 0)
+	if (opt < 0) {
 		/* We might like to report bad mount options here, but
 		 * traditionally we've ignored all mount options, so we'd
 		 * better continue to ignore non-existing options for bpf.
 		 */
-		return opt == -ENOPARAM ? 0 : opt;
+		if (opt == -ENOPARAM) {
+			if (strcmp(param->key, "source") == 0) {
+				if (param->type != fs_value_is_string)
+					return 0;
+				if (fc->source)
+					return 0;
+				fc->source = param->string;
+				param->string = NULL;
+			}
+
+			return 0;
+		}
+
+		return opt;
+	}
 
 	switch (opt) {
 	case OPT_MODE: