Quote mount point when resizing nilfs2 (#782681)

The current nilfs2 resizing code is safe because it never passes a user
influenced mount point into a command line.  Regardless, apply the same
simple rule to always quote mount points when used in command lines.

WARNING:
Nilfs-resize is broken and can't actually resize a file system mounted
with spaces in the mount point anyway!

    # mkdir "/tmp/File System Label"
    # mount -v -t nilfs2 /dev/sdb3 "/tmp/File System Label"
    mount.nilfs2: started nilfs_cleanerd
    # nilfs-resize -v -y /dev/sdb3
    Error: cannot open NILFS on /dev/sdb3.
    # echo $?
    1
    # strace nilfs-resize -v -y /dev/sdb3
    ...
    stat("/dev/sdb3", {st_mode=S_IFBLK|0660, st_rdev=makedev(8, 19), ...}) = 0
    open("/dev/sdb3", O_RDONLY)             = 3
    ...
    open("/proc/mounts", O_RDONLY)          = 4
    read(4, "sysfs /sys sysfs rw,seclabel,nos"..., 1024) = 1024
    ...
    close(4)                                = 0
    open("/tmp/File\\040System\\040Label", O_RDONLY) = -1 ENOENT (No such file or directory)
    ...
    # fgrep /dev/sd /proc/mounts
    /dev/sda3 / ext4 rw,seclabel,relatime,data=ordered 0 0
    /dev/sda1 /boot ext4 rw,seclabel,relatime,data=ordered 0 0
    /dev/sdb3 /tmp/File\040System\040Label nilfs2 rw,relatime 0 0

So it looks like nilfs-resize (or a library it uses) can't decode the
octal characters '\040' used to encode spaces in the mount point as
reported in /proc/mounts.

Bug 782681 - btrfs partitions mounted with whitespace cannot be resized