xtensa: use strscpy to copy strings

The strlcpy should not be used because it doesn't limit the source
length. So that it will lead some potential bugs.

But the strscpy doesn't require reading memory from the src string
beyond the specified "count" bytes, and since the return value is
easier to error-check than strlcpy()'s. In addition, the implementation
is robust to the string changing out from underneath it, unlike the
current strlcpy() implementation.

Thus, replace strlcpy with strscpy.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Message-Id: <20211220084756.955307-1-wangborong@cdjrlc.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>