btrfs: don't error on large length value in btrfs exent-same

Instead we just dedupe the maximum allowed. That way userspace doesn't have
to worry about an arbitrary length limit. Since bytes deduped is returned
from the ioctl, userspace will always have the knowledge that it can ask for
another dedupe.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>