fix(vfs): wait for pipe space before file-to-pipe splice read

[fslongjin]

Summary

• align file-to-pipe splice with Linux ordering by waiting for output pipe space before reading from the input file
• add a pipe helper that preserves PIPE_BUF atomic write semantics for blocking splice writes
• avoid advancing/reading from the input file before an interruptible pipe wait can complete

Validation

• make kernel
• make fmt
• TCG guest: ./splice_test --gtest_filter=*Concurrent* passed, SpliceTest.FromPipeWithConcurrentIo completed in ~1s
• TCG guest: ./splice_test --gtest_filter=*SmallCapacity*:*Concurrent* passed
• TCG guest: ./gvisor-test-runner splice_test passed, 27/27 tests, SpliceTest.FromPipeWithConcurrentIo completed in ~3.4s, full splice_test in ~4.8s

[fslongjin]

更新：在原有 file->pipe splice 等待顺序修复之外，补了一层 RwSem 等待者公平性修复，避免 madvise(MADV_DONTNEED) 这类短写锁循环在释放后立即重抢 address_space 写锁，导致已经排队的 splice/read 路径长期拿不到读锁。

本轮验证：

• make kernel 通过
• make fmt 通过（包含 kernel clippy）
• make -C user/apps/tests/dunitest build-suites 通过
• Linux 本地执行 splice_concurrent_io_test --gtest_filter=SpliceConcurrentIo.FileToPipeNotStarvedByMadvise 通过
• DragonOS guest 中执行 gVisor splice_test 通过：27/27 passed，原卡住的 SpliceTest.FromPipeWithConcurrentIo 未再超时
• DragonOS guest 中新增 dunitest normal/splice_concurrent_io 通过

备注：全量 make test-dunit 当前仍失败在仓库已有无关 dunitest/内核问题（例如 fdatasync 未实现、pid_namespace_fork、udp_ipv6_send_semantics panic 等），但本次新增的 splice 并发回归用例已在该轮中通过。

[fslongjin]

已修复 CI 中 epoll_test 的 panic。

根因：上一版 RwSem 公平性修改让 read() 在“锁当前可读、但队列里已有 waiter”的情况下也可能主动睡眠一次。EpollTest.DoubleEpollOneShot 路径里 sched_getparam -> UserBuffer::write_one -> user_accessible_len -> vm.read() 可能处在 preempt_count == 1 的上下文，原语义要求锁可读时不能 schedule；主动睡眠触发了 sched::schedule() 的 preempt_count == 0 断言。

修正：

• RwSem::read() / read_interruptible() 恢复为先 try_read()，只有真的被 writer/upgrader 阻塞时才进入等待队列。
• RwSem::write() / write_interruptible() 仍在普通可睡眠上下文中避免插队已排队 waiter，以保留对 madvise 短写锁循环饿死 splice 读侧的修复；如果当前 preempt_count != 0，则不因 waiter 存在而主动睡眠。

本轮验证：

• make kernel 通过
• make fmt 通过（包含 kernel clippy）
• 重新写入带 gVisor syscall tests 的镜像后，在 DragonOS guest 中执行：
  • epoll_test 通过：21 个测试，20 passed / 1 skipped，没有再 panic
  • splice_test 通过：27/27 passed，SpliceTest.FromPipeWithConcurrentIo 未超时

[fslongjin]

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