Fix buffer size warning for strcpy

Fix buffer size warning for strcpy

To meet requirements of Common Criteria certification vulnerability
assessment. Static code analysis has been run and found the following
error:
buffer_size_warning: Calling "strncpy" with a maximum size
argument of 16 bytes on destination array "ve->name" of
size 16 bytes might leave the destination string unterminated.
https://people.redhat.com/ncroxon/mdadm-4.2-rc2-scan-results.html

The change is to make the destination size to fit the allocated size.

V5:
Simplify the the strnlen call.

V4:
Code cleanup of the interim "if" statement.

V3: Doc change only:
The code change from filling ve->name with spaces to filling it with
null-terminated is to comform to the SNIA - Common RAID Disk Data
Format Specification. The format for VD_Name (ve->name) specifies
the field to be either ASCII or UNICODE. Bit 2 of the VD_Type field
MUST be used to determine the Unicode or ASCII format of this field.
If this field is not used, all bytes MUST be set to zero.

V2: Change from zero-terminated to zero-padded on memset and
change from using strncpy to memcpy, feedback from Neil Brown.

Tested-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Nigel Croxon <ncroxon@redhat.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>