nimble/host: Add offset in GATT server long read context

This allows application to easily determine if client is performing
GATT Long Read operation.

Author: vandy

nimble/host/include/host/ble_gatt.h

@@ -960,6 +960,12 @@ struct ble_gatt_access_ctxt {
          */
         const struct ble_gatt_dsc_def *dsc;
     };
+
+    /**
+     * An offset in case of BLE_ATT_OP_READ_BLOB_REQ.
+     * If the value is greater than zero it's an indication of a long attribute read.
+     */
+    uint16_t offset;
 };
 
 /**

nimble/host/src/ble_gatts.c

@@ -416,6 +416,7 @@ ble_gatts_chr_val_access(uint16_t conn_handle, uint16_t attr_handle,
 
     gatt_ctxt.op = ble_gatts_chr_op(att_op);
     gatt_ctxt.chr = chr_def;
+    gatt_ctxt.offset = offset;
 
     ble_gatts_chr_inc_val_stat(gatt_ctxt.op);
     rc = ble_gatts_val_access(conn_handle, attr_handle, offset, &gatt_ctxt, om,