nfs: Enclose hostname in brackets when needed in nfs_do_root_mount

nfs: Enclose hostname in brackets when needed in nfs_do_root_mount

commit 98a2139f4f4d7b5fcc3a54c7fddbe88612abed20 upstream.

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>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>