commit d74c6d51 upstream.

__bio_for_each_segment() iterates bvecs from the specified index
instead of bio->bv_idx.  Currently, the only usage is to walk all the
bvecs after the bio has been advanced by specifying 0 index.

For immutable bvecs, we need to split these apart;
bio_for_each_segment() is going to have a different implementation.
This will also help document the intent of code that's using it -
bio_for_each_segment_all() is only legal to use for code that owns the
bio.
Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
CC: Neil Brown <neilb@suse.de>
CC: Boaz Harrosh <bharrosh@panasas.com>
[bwh: Backported to 3.2: drop inapplicable change to drivers/block/rbd.c.
 This is a prerequisite for commit 35dc2483 'sg: Fix user memory
 corruption when SG_IO is interrupted by a signal']
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>