dm thin: fix stacked bi_next usage

Avoid using the bi_next field for the holder of a cell when deferring
bios because a stacked device below might change it.  Store the
holder in a new field in struct cell instead.

When a cell is created, the bio that triggered creation (the holder) was
added to the same bio list as subsequent bios.  In some cases we pass
this holder bio directly to devices underneath.  If those devices use
the bi_next field there will be trouble...

This also simplifies some code that had to work out which bio was the
holder.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>