erts: Cap the adaptive esock rcvbuf at 256 KB

Draining a local bulk stream on an M2 Pro is flat or slightly better
with a 128 KB or 256 KB cap (7.4-7.6 GB/s) than with 1 MB (6.0 GB/s),
so past the point where the buffer amortises the syscall the larger
memcpy only costs. 256 KB keeps 2x headroom over the measured knee
for links with more latency than loopback.

Author: lpgauth

erts/emulator/nifs/unix/unix_socket_syncio.c

@@ -214,7 +214,7 @@
  * the configured size once an EWMA of the read sizes falls below a
  * quarter of it.
  */
-#define ESSIO_RECV_ADAPT_BUFFER_MAX     (1 << 20)
+#define ESSIO_RECV_ADAPT_BUFFER_MAX     (1 << 18)
 #define ESSIO_RECV_ADAPT_EWMA_SHIFT     3
 // #define sock_close_event(e)             /* do nothing */
 #define sock_connect(s, addr, len)      connect((s), (addr), (len))