Add two checks for potential buffer cache races.

Add two checks for potential buffer cache races.

"Warning buffer %p (vp %p lblkno %d) was recycled"
    Occurs if a buffer is recycled unexpectedly.  The code will print this
    warning and retry if it detects the case.

"Warning invalid buffer %p (vp %p lblkno %d) did not have cleared b_blkno cache"
    Occurs if a B_INVAL buffer's b_blkno cache has not been reset.  The
    code will reset the cache if it detects this case.