nfs: Do not allow multiple mounts on same mountpoint when using -o noac

Do not allow multiple mounts on same mountpoint when using -o noac

When you normally attempt to mount a share twice on the same mountpoint,
a check in do_add_mount causes it to return an error

# mount localhost:/nfsv3 /mnt
# mount localhost:/nfsv3 /mnt
mount.nfs: /mnt is already mounted or busy

However when using the option 'noac', the user is able to mount the same
share on the same mountpoint multiple times. This happens because a
share mounted with the noac option is automatically assigned the 'sync'
flag MS_SYNCHRONOUS in nfs_initialise_sb(). This flag is set after the
check for already existing superblocks is done in sget(). The check for
the mount flags in nfs_compare_mount_options() does not take into
account the 'sync' flag applied later on in the code path. This means
that when using 'noac', a new superblock structure is assigned for every
new mount of the same share and multiple shares on the same mountpoint
are allowed.

ie.
# mount -onoac localhost:/nfsv3 /mnt
can be run multiple times.

The patch checks for noac and assigns the sync flag before sget() is
called to obtain an already existing superblock structure.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>