RAID5 is calling copy_data() under sh->lock.  But copy_data() does kmap(),
which can sleep.

The best fix is to use kmap_atomic() in there.  It is faster than kmap() and
does not block.

The patch removes the unused bio_kmap() and replaces __bio_kmap() with
__bio_kmap_atomic().  I think it's best to withdraw the sleeping-and-slow
bio_kmap() from the kernel API before someone else tries to use it.


Also, I notice that bio_kmap_irq() was using local_save_flags().  This is a
bug - local_save_flags() does not disable interrupts.  Converted that to
local_irq_save().  These names are terribly chosen.

This patch was acked by Jens and Neil.