LinuxKPI: add memset_startat macro

Add a memset_startat() macro which sets a pattern from a struct member
to the end of the struct.   Needed by a wireless driver.

MFC after:	3 days
Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D37389

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

@@ -249,4 +249,12 @@ memcpy_and_pad(void *dst, size_t dstlen, const void *src, size_t len, int ch)
 	}
 }
 
+#define	memset_startat(ptr, bytepat, smember)				\
+({									\
+	uint8_t *_ptr = (uint8_t *)(ptr);				\
+	int _c = (int)(bytepat);					\
+	size_t _o = offsetof(typeof(*(ptr)), smember);			\
+	memset(_ptr + _o, _c, sizeof(*(ptr)) - _o);			\
+})
+
 #endif	/* _LINUXKPI_LINUX_STRING_H_ */