Kernel: Only lock file descriptor table once in sys$pipe()

Instead of locking it twice, we now frontload all the work that doesn't
touch the fd table, and then only lock it towards the end of the
syscall.

The benefit here is simplicity. The downside is that we do a bit of
unnecessary work in the EMFILE error case, but we don't need to optimize
that case anyway.

Kernel/Syscalls/pipe.cpp

@@ -21,15 +21,6 @@ ErrorOr<FlatPtr> Process::sys$pipe(Userspace<int*> pipefd, int flags)
     u32 fd_flags = (flags & O_CLOEXEC) ? FD_CLOEXEC : 0;
     auto fifo = TRY(FIFO::try_create(uid()));
 
-    ScopedDescriptionAllocation reader_fd_allocation;
-    ScopedDescriptionAllocation writer_fd_allocation;
-
-    TRY(m_fds.with_exclusive([&](auto& fds) -> ErrorOr<void> {
-        reader_fd_allocation = TRY(fds.allocate());
-        writer_fd_allocation = TRY(fds.allocate());
-        return {};
-    }));
-
     auto reader_description = TRY(fifo->open_direction(FIFO::Direction::Reader));
     auto writer_description = TRY(fifo->open_direction(FIFO::Direction::Writer));
 
@@ -41,6 +32,9 @@ ErrorOr<FlatPtr> Process::sys$pipe(Userspace<int*> pipefd, int flags)
     }
 
     TRY(m_fds.with_exclusive([&](auto& fds) -> ErrorOr<void> {
+        auto reader_fd_allocation = TRY(fds.allocate());
+        auto writer_fd_allocation = TRY(fds.allocate());
+
         fds[reader_fd_allocation.fd].set(move(reader_description), fd_flags);
         fds[writer_fd_allocation.fd].set(move(writer_description), fd_flags);