bcachefs: preserve device path as device name

Various userspace scripts/tools may expect mount entries in
/proc/mounts to reflect the device path names used to mount the
associated filesystem. bcachefs seems to normalize the device path
to the underlying device name based on the block device. This
confuses tools like fstests when the test devices might be lvm or
device-mapper based.

The default behavior for show_vfsmnt() appers to be to use the
string passed to alloc_vfsmnt(), so tweak bcachefs to copy the path
at device superblock read time and to display it via
->show_devname().

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1667,8 +1667,7 @@ static int bch2_show_devname(struct seq_file *seq, struct dentry *root)
 		if (!first)
 			seq_putc(seq, ':');
 		first = false;
-		seq_puts(seq, "/dev/");
-		seq_puts(seq, ca->name);
+		seq_puts(seq, ca->disk_sb.sb_name);
 	}
 
 	return 0;

fs/bcachefs/super-io.c

@@ -166,6 +166,7 @@ void bch2_free_super(struct bch_sb_handle *sb)
 	if (!IS_ERR_OR_NULL(sb->bdev))
 		blkdev_put(sb->bdev, sb->holder);
 	kfree(sb->holder);
+	kfree(sb->sb_name);
 
 	kfree(sb->sb);
 	memset(sb, 0, sizeof(*sb));
@@ -675,6 +676,10 @@ retry:
 	if (!sb->holder)
 		return -ENOMEM;
 
+	sb->sb_name = kstrdup(path, GFP_KERNEL);
+	if (!sb->sb_name)
+		return -ENOMEM;
+
 #ifndef __KERNEL__
 	if (opt_get(*opts, direct_io) == false)
 		sb->mode |= BLK_OPEN_BUFFERED;

fs/bcachefs/super_types.h

@@ -5,6 +5,7 @@
 struct bch_sb_handle {
 	struct bch_sb		*sb;
 	struct block_device	*bdev;
+	char			*sb_name;
 	struct bio		*bio;
 	void			*holder;
 	size_t			buffer_size;