Not over-allocate client query buffer when reading large objects. (#5954)

In response to large client query buffer optimization introduced in 1898e6c. The calculation of the amount of
remaining bytes we need to write to the query buffer was calculated wrong, as a result we are unnecessarily
growing the client query buffer by sdslen(c->querybuf) always. This fix corrects that behavior.

Please note the previous behavior prior to the before-mentioned change was correctly calculating the remaining
additional bytes, and this change makes that calculate to be consistent.

Useful context, the argument of size `ll` starts at qb_pos (which is now the beginning of the sds), but much of it
may have already been read from the socket, so we only need to grow the sds for the remainder of it.

src/networking.c

@@ -1851,7 +1851,7 @@ int processMultibulkBuffer(client *c) {
                     c->qb_pos = 0;
                     /* Hint the sds library about the amount of bytes this string is
                      * going to contain. */
-                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2);
+                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-sdslen(c->querybuf));
                 }
             }
             c->bulklen = ll;