mdadm: set ident.devname if applicable

mdadm: set ident.devname if applicable

This patch tries to propagate the usage of struct mddev_ident for cmdline
where it is applicable. To avoid regression, this value is derived
from devlist->devname for applicable modes only.
As a result, the whole structure is passed to some functions. It produces
some changes for Build, Create and Assemble.
No functional changes intended.

The goal of the change is to unify devname validation which is done in
next patches.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>