Fix various bugs with aligned reads in RAID5.

It is possible for raid5 to be sent a bio that is too big
for an underlying device.  So if it is a READ that we
pass stright down to a device, it will fail and confuse
RAID5.

So in 'chunk_aligned_read' we check that the bio fits within the
parameters for the target device and if it doesn't fit, fall back
on reading through the stripe cache and making lots of one-page
requests.

Note that this is the earliest time we can check against the device
because earlier we don't have a lock on the device, so it could change
underneath us.

Also, the code for handling a retry through the cache when a read
fails has not been tested and was badly broken.  This patch fixes that
code.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>