LwIP: clamp reads to a value that can be expressed in 16 bits

There is a bug in the LwIP VFS plugin, the chained buffers used by Lwip
use sizes expressed in sixteen bits, and under conditions such as a read
of 1<<16 the higher bits are lost and the plugin performs a zero length
read, and the application interprets this as a closed connection.

Fix #2947

repos/libports/src/lib/vfs/lwip/vfs.cc

@@ -1775,6 +1775,12 @@ class Lwip::File_system final : public Vfs::File_system
 		                                  char *dst, file_size count,
 		                                  file_size &out_count) override
 		{
+			/*
+			 * LwIP buffer operations are limited to sizes that
+			 * can be expressed in sixteen bits
+			 */
+			count = Genode::min(count, 0xffffU);
+
 			out_count = 0;
 
 			if ((!vfs_handle) ||