xfstests: handle new mkfs.btrfs -f option cleanly

xfstests: handle new mkfs.btrfs -f option cleanly

I added an "-f" option to mkfs.btrfs to force overwrite
of an existing filesystem.  Now on an xfstests run, new
mkfs.btrfs requires it, and old mkfs.btrfs cannot accept
it.

So, add a helper which works out whether -f is needed,
and add it to the MKFS_BTRFS_PROG env. var as necessary,
so that it is an always-included option during the tests.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>