btrfs-progs: Fix number of arguments check of 'btrfs fi df'

btrfs-progs: Fix number of arguments check of 'btrfs fi df'

'btrfs fi df' needs exactly one arguments as mount option,
but as 3.17 we can run 'btrfs fi df' without any argument,
and it will error as "ERROR: can't access '%s'" which means
the argument number does not do what it should.

The bug is caused by manually modify the optind and use check_argc_max()
instead of the original check_argc_exact().

This patch fixes it by not modifying the optind and use check_argc_exact()
again.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>