nfs: Enclose hostname in brackets when needed in nfs_do_root_mount

When hostname contains colon (e.g. when it is an IPv6 address) it needs
to be enclosed in brackets to make parsing of NFS device string possible.
Fix nfs_do_root_mount() to enclose hostname properly when needed. NFS code
actually does not need this as it does not parse the string passed by
nfs_do_root_mount() but the device string is exposed to userspace in
/proc/mounts.

CC: Josh Boyer <jwboyer@redhat.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -2767,10 +2767,14 @@ static struct vfsmount *nfs_do_root_mount(struct file_system_type *fs_type,
 	char *root_devname;
 	size_t len;
 
-	len = strlen(hostname) + 3;
+	len = strlen(hostname) + 5;
 	root_devname = kmalloc(len, GFP_KERNEL);
 	if (root_devname == NULL)
 		return ERR_PTR(-ENOMEM);
+	/* Does hostname needs to be enclosed in brackets? */
+	if (strchr(hostname, ':'))
+		snprintf(root_devname, len, "[%s]:/", hostname);
+	else
 		snprintf(root_devname, len, "%s:/", hostname);
 	root_mnt = vfs_kern_mount(fs_type, flags, root_devname, data);
 	kfree(root_devname);