Kernel: FileDescription::is_directory() should not assert !is_fifo()

I have no idea why this was here. It makes no sense. If you're trying
to find out if something is a directory, why wouldn't you be allowed to
ask that about a FIFO? :^)

Thanks to Brandon for spotting this!

Also, while we're here, cache the directory state in a bool member so
we don't have to keep fetching inode metadata when checking this
repeatedly. This is important since sys$read() now calls it.

Kernel/FileSystem/FileDescription.cpp

@@ -34,6 +34,7 @@ FileDescription::FileDescription(File& file)
         m_inode = static_cast<InodeFile&>(file).inode();
     if (is_socket())
         socket()->attach(*this);
+    m_is_directory = metadata().is_directory();
 }
 
 FileDescription::~FileDescription()
@@ -138,19 +139,15 @@ ByteBuffer FileDescription::read_entire_file()
     return m_inode->read_entire(this);
 }
 
-bool FileDescription::is_directory() const
-{
-    ASSERT(!is_fifo());
-    return metadata().is_directory();
-}
 
 ssize_t FileDescription::get_dir_entries(u8* buffer, ssize_t size)
 {
+    if (!is_directory())
+        return -ENOTDIR;
+
     auto metadata = this->metadata();
     if (!metadata.is_valid())
         return -EIO;
-    if (!metadata.is_directory())
-        return -ENOTDIR;
 
     int size_to_allocate = max(PAGE_SIZE, metadata.size);
 

Kernel/FileSystem/FileDescription.h

@@ -44,7 +44,7 @@ public:
 
     String absolute_path() const;
 
-    bool is_directory() const;
+    bool is_directory() const { return m_is_directory; }
 
     File& file() { return *m_file; }
     const File& file() const { return *m_file; }
@@ -115,6 +115,7 @@ private:
     u32 m_file_flags { 0 };
 
     bool m_is_blocking { true };
+    bool m_is_directory { false };
     bool m_should_append { false };
     FIFO::Direction m_fifo_direction { FIFO::Direction::Neither };
 };