linuxkpi: Add strscpy_pad() to <linux/string.h>

It's the same as `strscpy()` except that it fills the rest of the
destination buffer with zeroes if the source buffer is shorter.

Reviewed by:	manu
Approved by:	manu

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

@@ -207,6 +207,15 @@ strscpy(char* dst, const char* src, size_t len)
 	return (-E2BIG);
 }
 
+static inline ssize_t
+strscpy_pad(char* dst, const char* src, size_t len)
+{
+
+	bzero(dst, len);
+
+	return (strscpy(dst, src, len));
+}
+
 static inline void *
 memset32(uint32_t *b, uint32_t c, size_t len)
 {