(bread): For a read that is contained entirely within a block, never copy

out more than size bytes to the caller's buffer.

PR:		bin/111191
Reviewed by:	pb
MFC after:	1 week

sbin/dump/traverse.c

@@ -925,7 +925,7 @@ loop:
 			cnt = cread(diskfd, tmpbuf, secsize, offset - base);
 			if (cnt != secsize)
 				goto bad;
-			xfer = secsize - base;
+			xfer = MIN(secsize - base, size);
 			offset += xfer;
 			bytes -= xfer;
 			resid = bytes % secsize;