dm: fix dm_merge_bvec regression on 32 bit systems

dm: fix dm_merge_bvec regression on 32 bit systems

A DM regression on 32 bit systems was reported against v4.2-rc3 here:
https://lkml.org/lkml/2015/7/29/401

Fix this by reverting both commit 711a0055 ("dm: fix casting bug in
dm_merge_bvec()") and 6f41cc9d ("dm: improve documentation and code
clarity in dm_merge_bvec").  This combined revert is done to eliminate
the possibility of a partial revert in stable@ kernels.

In hindsight the correct fix, at the time 711a0055 was applied to fix
the regression that 6f41cc9d introduced, should've been to simply revert
6f41cc9d.

Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
Tested-by: Adam Williamson <awilliam@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.19+