Close a race which could allow for privilege escalation by users with DRI

    privileges on Radeon hardware. Essentially, a malicious program could
    submit a packet containing an offset (possibly in main memory) to be
    rendered from/to, while a separate thread switched that offset in
    userspace rapidly between a valid value and an invalid one.
    radeon_check_and_fixup_offset() would pull the offset in from user
    space, check it, and spit it back out to user space to be copied in
    later by the emit code. It would sometimes catch the bad value, but
    sometimes the malicious program could modify it after the check and get
    an invalid offset rendered from/to.
Fix this by allocating a temporary buffer and copying the data in at once.
    While here, make the cliprects stuff not do the VERIFYAREA_READ and
    COPY_FROM_USER_UNCHECKED gymnastics, avoiding a lock order reversal on
    FreeBSD. Performance impact is negligible -- no difference on r200 to
    ~1% improvement on rv200 in quake3 tests (P4 1Ghz, demofour at
    1024x768, n=4 or 5).