GATT multi read: account for length for variable length read

The response will contain length, then the bytes with actual content.
Account for the 2 byte length field when computing how much data can be
squezed into a packet.

Bug: 227618988
Test: compilation
Change-Id: I6ddee3e5c50ce395fa4cc1b538d256a406a415f0

stack/gatt/gatt_sr.cc

diff --git a/stack/gatt/gatt_sr.cc b/stack/gatt/gatt_sr.cc
index c0aedfa..39c7125 100644
--- a/stack/gatt/gatt_sr.cc
+++ b/stack/gatt/gatt_sr.cc
@@ -172,6 +172,9 @@
 
     if (p_rsp != NULL) {
       total_len = (p_buf->len + p_rsp->attr_value.len);
+      if (p_cmd->multi_req.variable_len) {
+        total_len += 2;
+      }
 
       if (total_len > mtu) {
         /* just send the partial response for the overflow case */