xfstests: automatically add -F to xfs_io on non-xfs

xfstests: automatically add -F to xfs_io on non-xfs

The -F flag to xfs_io originally enabled it to operate on non-xfs
filesystems.  This restriction was removed upstream in favor of
gracefully failing on the handful of operations that actually
required xfs, and the option was deprecated.

However, xfstests is still used on distros with older xfsprogs, and
so "xfs_io -F" was necessary throughout xfstests.

Simplify this by appending -F to XFS_IO_PROG when it's needed -
i.e. if we're using old xfsprogs on a non-xfs filesystem.

This will eliminate errors when new tests leave out the -F, and
if and when -F is finally removed, there will be one central
location in xfstests to update.

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