bcachefs: New magic number

Add a new bcachefs-specific magic number for the superblock, instead of
continuing to use the old bcache magic number3
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -1514,7 +1514,7 @@ struct bch_sb_layout {
  * @version_min	- Oldest metadata version this filesystem contains; so we can
  *		  safely drop compatibility code and refuse to mount filesystems
  *		  we'd need it for
- * @magic	- identifies as a bcachefs superblock (BCACHE_MAGIC)
+ * @magic	- identifies as a bcachefs superblock (BCHFS_MAGIC)
  * @seq		- incremented each time superblock is written
  * @uuid	- used for generating various magic numbers and identifying
  *                member devices, never changes

fs/bcachefs/super-io.c

@@ -809,6 +809,11 @@ int bch2_write_super(struct bch_fs *c)
 	closure_init_stack(cl);
 	memset(&sb_written, 0, sizeof(sb_written));
 
+	if (c->opts.version_upgrade) {
+		c->disk_sb.sb->magic = BCHFS_MAGIC;
+		c->disk_sb.sb->layout.magic = BCHFS_MAGIC;
+	}
+
 	le64_add_cpu(&c->disk_sb.sb->seq, 1);
 
 	if (test_bit(BCH_FS_ERROR, &c->flags))