bcachefs: Fix a kasan splat in bch2_dev_add()

This fixes a use after free - mi is dangling after the resize call.

Additionally, resizing the device's member info section was useless - we
were attempting to preallocate the space required before adding it to
the filesystem superblock, but there's other sections that we should
have been preallocating as well for that to work.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -1622,16 +1622,6 @@ int bch2_dev_add(struct bch_fs *c, const char *path)
 		goto err_unlock;
 	}
 
-	mi = bch2_sb_field_get(ca->disk_sb.sb, members_v2);
-
-	if (!bch2_sb_field_resize(&ca->disk_sb, members_v2,
-				le32_to_cpu(mi->field.u64s) +
-				sizeof(dev_mi) / sizeof(u64))) {
-		ret = -BCH_ERR_ENOSPC_sb_members;
-		bch_err_msg(c, ret, "setting up new superblock");
-		goto err_unlock;
-	}
-
 	if (dynamic_fault("bcachefs:add:no_slot"))
 		goto no_slot;
 
@@ -1645,6 +1635,8 @@ int bch2_dev_add(struct bch_fs *c, const char *path)
 
 have_slot:
 	nr_devices = max_t(unsigned, dev_idx + 1, c->sb.nr_devices);
+
+	mi = bch2_sb_field_get(c->disk_sb.sb, members_v2);
 	u64s = DIV_ROUND_UP(sizeof(struct bch_sb_field_members_v2) +
 			    le16_to_cpu(mi->member_bytes) * nr_devices, sizeof(u64));