mdadm: define ident_set_devname()

mdadm: define ident_set_devname()

Use dedicated set method for ident->devname. Now, devname validation
is done early for modes where device is created (Build, Create and
Assemble). The rules, used for devname validation are derived from
config file.

It could cause regression with execeptional cases where existing device
has name which doesn't match criteria for Manage and Grow modes. It is
low risk and those modes are not omitted from early devname validation.
Use can used main numbered devnode to avoid this problem.
Messages exposed to user are changed so it might cause a regression
in negative scenarios. Error codes are not changed.

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