LinuxKPI: Implement strscpy

strscpy copies the src string, or as much of it as fits, into the dst
buffer.  The dst buffer is always NUL terminated, unless it's zero-sized.
strscpy returns the number of characters copied (not including the
trailing NUL) or -E2BIG if len is 0 or src was truncated.

Currently drm-kmod replaces strscpy with strncpy that is not quite
correct as strncpy does not NUL-terminate truncated strings and returns
different values on exit.

Reviewed by:	hselasky, imp, manu
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D31005

sys/compat/linuxkpi/common/include/linux/string.h

@@ -167,4 +167,20 @@ str_has_prefix(const char *str, const char *prefix)
 	return (strncmp(str, prefix, len) == 0 ? len : 0);
 }
 
+static inline ssize_t
+strscpy(char* dst, const char* src, size_t len)
+{
+	size_t i;
+
+	if (len <= INT_MAX) {
+		for (i = 0; i < len; i++)
+			if ('\0' == (dst[i] = src[i]))
+				return ((ssize_t)i);
+		if (i != 0)
+			dst[--i] = '\0';
+	}
+
+	return (-E2BIG);
+}
+
 #endif					/* _LINUX_STRING_H_ */