Kernel+LibC: Add msync() system call

This allows userspace to trigger a full (FIXME) flush of a shared file
mapping to disk. We iterate over all the mapped pages in the VMObject
and write them out to the underlying inode, one by one. This is rather
naive, and there's lots of room for improvement.

Note that shared file mappings are currently not possible since mmap()
returns ENOTSUP for PROT_WRITE+MAP_SHARED. That restriction will be
removed in a subsequent commit. :^)

Kernel/Memory/SharedInodeVMObject.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <Kernel/FileSystem/Inode.h>
+#include <Kernel/Locking/Spinlock.h>
 #include <Kernel/Memory/SharedInodeVMObject.h>
 
 namespace Kernel::Memory {
@@ -34,4 +35,22 @@ SharedInodeVMObject::SharedInodeVMObject(SharedInodeVMObject const& other)
 {
 }
 
+ErrorOr<void> SharedInodeVMObject::sync()
+{
+    SpinlockLocker locker(m_lock);
+
+    for (size_t page_index = 0; page_index < page_count(); ++page_index) {
+        auto& physical_page = m_physical_pages[page_index];
+        if (!physical_page)
+            continue;
+
+        u8 page_buffer[PAGE_SIZE];
+        MM.copy_physical_page(*physical_page, page_buffer);
+
+        TRY(m_inode->write_bytes(page_index * PAGE_SIZE, PAGE_SIZE, UserOrKernelBuffer::for_kernel_buffer(page_buffer), nullptr));
+    }
+
+    return {};
+}
+
 }