btrfs: fail if fstrim_range->start == U64_MAX

We've always been failing generic/260 because it's testing things we
actually don't care about and thus won't fail for.  However we probably
should fail for fstrim_range->start == U64_MAX since we clearly can't
trim anything past that.  This in combination with an update to
generic/260 will allow us to pass this test properly.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>