mdadm: Add define for magic +9 in version string

mdadm: Add define for magic +9 in version string

To have it better understandable add definitions for that.
Definition is placed in md_common.h because it is corresponding to both
mdstat and sysfs implementations. Probably more shared properties will
be defined in the future.

The MD_EXTERNAL_PREFIX_LEN and MD_VERSION_BLOCKED_IDX are the same
but are defined for different purpose. One is specyfing size
of string but the second is specyfing index in the array.

Also, replaces magic "external" used in code.

No functional changes intended.

Signed-off-by: Mariusz Tkaczyk <mtkaczyk@kernel.org>