scatterlist: don't BUG when we can trivially return a proper error.

scatterlist: don't BUG when we can trivially return a proper error.

There is absolutely no reason to crash the kernel when we have a perfectly
good return value already available to use for conveying failure status.

Let's return an error code instead of crashing the kernel: that sounds
like a much better plan.

Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>