btrfs-progs: Fix a regression that btrfs filesystem label doesn't work

btrfs-progs: Fix a regression that btrfs filesystem label doesn't work

The number of arguments which is allowed to pass became wrong
from the following commit.

commit 176aeca9a148c5e29de0 ("btrfs-progs: add getopt stubs where needed")

* actual result

  ===========================================================
  # ./btrfs prop get /btrfs label
  label=foo
  # ./btrfs fi label /btrfs
  btrfs filesystem label: too few arguments
  usage: btrfs filesystem label [<device>|<mount_point>] [<newlabel>]

      Get or change the label of a filesystem

      With one argument, get the label of filesystem on <device>.
      If <newlabel> is passed, set the filesystem label to <newlabel>.

  # ./btrfs fi label /btrfs bar
  foo
  # ./btrfs prop get /btrfs label
  label=foo
  ===========================================================

* expected result

  ===========================================================
  # ./btrfs prop get /btrfs label
  label=foo
  # ./btrfs fi label /btrfs
  foo
  # ./btrfs fi label /btrfs bar
  # ./btrfs prop get /btrfs label
  label=bar
  ===========================================================

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>