nfs: Enable JUKEBOX backpressure threshold on macOS

Summary:
Change the `fschannel:max-inflight-requests` default from 0 (disabled) to
`10 * CPU count` on macOS, keeping it disabled on other platforms. This
enables NFS3ERR_JUKEBOX backpressure by default on macOS, where NFS is the
mount protocol.

The threshold of 10× CPU count provides 24× headroom over steady-state
concurrency (~4.3) and 2.4× over ODS p99 estimates (~42), while keeping
drain time trivial (13.6ms at 100 requests vs 20s kernel timeout). False
positives are expensive (1s retry penalty from MUTEJUKEBOX backoff vs 139µs
avg service time), so the threshold errs on the generous side. The threshold
scales naturally across Mac hardware: 80 on M1, 100 on M1 Pro, 120 on M2
Pro, 160 on M3 Max.

Reviewed By: giorgidze

Differential Revision: D100398611

fbshipit-source-id: c4d12ce644be8a10080193c5cb5e8ec98fa613f4

Author: MichaelCuevas

eden/fs/config/EdenConfig.h

@@ -1138,10 +1138,12 @@ class EdenConfig : private ConfigSettingManager {
    * available, NFS returns NFS3ERR_JUKEBOX causing the client to retry.
    * This value is also used as the threshold for logging high request counts.
    * When set to 0, no limit is enforced and no logging will occur.
+   * Defaults to 10× CPU count on macOS (~100 on typical Macs), 0 (disabled)
+   * on other platforms.
    */
   ConfigSetting<uint64_t> maxFsChannelInflightRequests{
       "fschannel:max-inflight-requests",
-      0,
+      folly::kIsApple ? folly::available_concurrency() * 10 : 0,
       this};
 
   ConfigSetting<std::chrono::nanoseconds> highFsRequestsLogInterval{