mdadm: remove POSIX check

mdadm: remove POSIX check

Neil Brown in #159 pointed that mdadm should been keep in base utility
style, allowing much more with no strict limitations until absolutely
necessary to prevent crashes.

This view, supported with regression #160 caused by POSIX portable
character set requirement leads me to revert it.

Revert the POSIX portable character set verification of name and
devname. Make it IMSM only.

Fixes: e2eb503bd797 ("mdadm: Follow POSIX Portable Character Set")
Signed-off-by: Mariusz Tkaczyk <mtkaczyk@kernel.org>