Kernel: Add InodeFile, a File subclass for regular files.

Finally everything that can be held by a FileDescriptor actually inherits
from the File class.

Kernel/FileSystem/InodeFile.cpp

@@ -0,0 +1,55 @@
+#include <Kernel/FileSystem/InodeFile.h>
+#include <Kernel/FileSystem/Inode.h>
+#include <Kernel/FileSystem/FileDescriptor.h>
+#include <Kernel/FileSystem/VirtualFileSystem.h>
+#include <Kernel/Process.h>
+
+InodeFile::InodeFile(Retained<Inode>&& inode)
+    : m_inode(move(inode))
+{
+}
+
+InodeFile::~InodeFile()
+{
+}
+
+ssize_t InodeFile::read(FileDescriptor& descriptor, byte* buffer, ssize_t count)
+{
+    return m_inode->read_bytes(descriptor.offset(), count, buffer, &descriptor);
+}
+
+ssize_t InodeFile::write(FileDescriptor& descriptor, const byte* data, ssize_t count)
+{
+    return m_inode->write_bytes(descriptor.offset(), count, data, &descriptor);
+}
+
+KResultOr<Region*> InodeFile::mmap(Process& process, LinearAddress preferred_laddr, size_t offset, size_t size, int prot)
+{
+    ASSERT(offset == 0);
+    // FIXME: If PROT_EXEC, check that the underlying file system isn't mounted noexec.
+    String region_name;
+#if 0
+    // FIXME: I would like to do this, but it would instantiate all the damn inodes.
+    region_name = absolute_path();
+#else
+    region_name = "Memory-mapped file";
+#endif
+    InterruptDisabler disabler;
+    auto* region = process.allocate_file_backed_region(preferred_laddr, size, inode(), move(region_name), prot & PROT_READ, prot & PROT_WRITE);
+    if (!region)
+        return KResult(-ENOMEM);
+    return region;
+}
+
+String InodeFile::absolute_path(FileDescriptor&) const
+{
+    auto path_or_error = VFS::the().absolute_path(const_cast<Inode&>(inode()));
+    if (path_or_error.is_error())
+        return { };
+    return path_or_error.value();
+}
+
+KResult InodeFile::truncate(off_t size)
+{
+    return m_inode->truncate(size);
+}