kboot: space_avail -- how much space exists from 'start' to end of segment

Sponsored by:		Netflix
Reviewed by:		tsoome
Differential Revision:	https://reviews.freebsd.org/D38313

stand/kboot/kboot.h

@@ -44,6 +44,7 @@ void remove_avail(uint64_t start, uint64_t end, uint64_t type);
 uint64_t first_avail(uint64_t align, uint64_t min_size, uint64_t type);
 void print_avail(void);
 bool populate_avail_from_iomem(void);
+uint64_t space_avail(uint64_t start);
 
 /* util.c */
 bool file2str(const char *fn, char *buffer, size_t buflen);

stand/kboot/seg.c

@@ -344,3 +344,22 @@ populate_avail_from_iomem(void)
 	close(fd);
 	return true;
 }
+
+/*
+ * Return the amount of space available in the segment that @start@ lives in,
+ * from @start@ to the end of the segment.
+ */
+uint64_t
+space_avail(uint64_t start)
+{
+	for (int i = 0; i < nr_seg; i++) {
+		if (start >= segs[i].start && start <= segs[i].end)
+			return segs[i].end - start;
+	}
+
+	/*
+	 * Properly used, we should never get here. Unsure if this should be a
+	 * panic or not.
+	 */
+	return 0;
+}