bcachefs: Fix creating snapshot with implict source

When creating a snapshot without specifying the source subvolume, we use
the subvolume containing the new snapshot.

Previously, this worked if the directory containing the new snapshot was
the subvolume root - but we were using the incorrect helper, and got a
subvolume ID of 0 when the parent directory wasn't the root of the
subvolume, causing an emergency read-only.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-ioctl.c

@@ -413,7 +413,7 @@ static long __bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
 
 	if ((arg.flags & BCH_SUBVOL_SNAPSHOT_CREATE) &&
 	    !arg.src_ptr)
-		snapshot_src.subvol = to_bch_ei(dir)->ei_inode.bi_subvol;
+		snapshot_src.subvol = inode_inum(to_bch_ei(dir)).subvol;
 
 	inode = __bch2_create(file_mnt_idmap(filp), to_bch_ei(dir),
 			      dst_dentry, arg.mode|S_IFDIR,