scsi: ibmvscsis: Ensure partition name is properly NUL terminated

While reviewing another part of the code, Kees noticed that the strncpy of the
partition name might not always be NUL terminated. Switch to using strscpy
which does this safely.

Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>