mdadm: refactor ident->name handling

mdadm: refactor ident->name handling

Create dedicated setter for name in mddev_ident and propagate it.
Following changes are made:
- move duplicated code from  config.c and mdadm.c into new function.
- Add error enum in mdadm.h.
- Use MD_NAME_MAX instead of hardcoded value in mddev_ident.
- Use secure functions.
- Add more detailed verification of the name.
- make error messages reusable for cmdline and config:
    - for cmdline, these are errors so use pr_err().
    - for config, these are just warnings, so use pr_info().

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