Ext2FS: Fail the mount if BGD table cache allocation fails

Instead of asserting if we can't allocate enough memory for a BGD table
cache, just fail the mount instead.

Kernel/FileSystem/Ext2FileSystem.cpp

@@ -141,8 +141,12 @@ bool Ext2FS::initialize()
 
     unsigned blocks_to_read = ceil_div(m_block_group_count * sizeof(ext2_group_desc), block_size());
     BlockIndex first_block_of_bgdt = block_size() == 1024 ? 2 : 1;
-    m_cached_group_descriptor_table = KBuffer::create_with_size(block_size() * blocks_to_read, Region::Access::Read | Region::Access::Write, "Ext2FS: Block group descriptors");
-    auto buffer = UserOrKernelBuffer::for_kernel_buffer(m_cached_group_descriptor_table.value().data());
+    m_cached_group_descriptor_table = KBuffer::try_create_with_size(block_size() * blocks_to_read, Region::Access::Read | Region::Access::Write, "Ext2FS: Block group descriptors");
+    if (!m_cached_group_descriptor_table) {
+        dbgln("Ext2FS: Failed to allocate memory for group descriptor table");
+        return false;
+    }
+    auto buffer = UserOrKernelBuffer::for_kernel_buffer(m_cached_group_descriptor_table->data());
     read_blocks(first_block_of_bgdt, blocks_to_read, buffer);
 
 #ifdef EXT2_DEBUG

Kernel/FileSystem/Ext2FileSystem.h

@@ -120,8 +120,8 @@ private:
 
     const ext2_super_block& super_block() const { return m_super_block; }
     const ext2_group_desc& group_descriptor(GroupIndex) const;
-    ext2_group_desc* block_group_descriptors() { return (ext2_group_desc*)m_cached_group_descriptor_table.value().data(); }
-    const ext2_group_desc* block_group_descriptors() const { return (const ext2_group_desc*)m_cached_group_descriptor_table.value().data(); }
+    ext2_group_desc* block_group_descriptors() { return (ext2_group_desc*)m_cached_group_descriptor_table->data(); }
+    const ext2_group_desc* block_group_descriptors() const { return (const ext2_group_desc*)m_cached_group_descriptor_table->data(); }
     void flush_block_group_descriptor_table();
     unsigned inodes_per_block() const;
     unsigned inodes_per_group() const;
@@ -170,7 +170,7 @@ private:
     unsigned m_block_group_count { 0 };
 
     mutable ext2_super_block m_super_block;
-    mutable Optional<KBuffer> m_cached_group_descriptor_table;
+    mutable OwnPtr<KBuffer> m_cached_group_descriptor_table;
 
     mutable HashMap<InodeIndex, RefPtr<Ext2FSInode>> m_inode_cache;