lib/tst_mkfs.c: fix -f option for mkfs.btrfs

In the earlier version of btrfs-progs, e.g. btrfs-progs in
RHEL6.5GA whose version is v0.20-rc1, mkfs.btrfs doesn't
support -f option. So it should be judged in advance before
the command is called.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>