Ext2FS: Support reading from file holes

It's perfectly valid for ext2 inodes to have blocks with index 0.
It means that no physical block was allocated for that area of an inode
and we should treat it as if it's filled with zeroes.

Fixes #6139.

Kernel/FileSystem/Ext2FileSystem.cpp

@@ -896,14 +896,19 @@ ssize_t Ext2FSInode::read_bytes(off_t offset, ssize_t count, UserOrKernelBuffer&
 
     for (auto bi = first_block_logical_index; remaining_count && bi <= last_block_logical_index; bi = bi.value() + 1) {
         auto block_index = m_block_list[bi.value()];
-        VERIFY(block_index.value());
         size_t offset_into_block = (bi == first_block_logical_index) ? offset_into_first_block : 0;
         size_t num_bytes_to_copy = min((off_t)block_size - offset_into_block, remaining_count);
         auto buffer_offset = buffer.offset(nread);
-        auto result = fs().read_block(block_index, &buffer_offset, num_bytes_to_copy, offset_into_block, allow_cache);
-        if (result.is_error()) {
-            dmesgln("Ext2FSInode[{}]::read_bytes(): Failed to read block {} (index {})", identifier(), block_index.value(), bi);
-            return result.error();
+        if (block_index.value() == 0) {
+            // This is a hole, act as if it's filled with zeroes.
+            if (!buffer_offset.memset(0, num_bytes_to_copy))
+                return -EFAULT;
+        } else {
+            auto result = fs().read_block(block_index, &buffer_offset, num_bytes_to_copy, offset_into_block, allow_cache);
+            if (result.is_error()) {
+                dmesgln("Ext2FSInode[{}]::read_bytes(): Failed to read block {} (index {})", identifier(), block_index.value(), bi);
+                return result.error();
+            }
         }
         remaining_count -= num_bytes_to_copy;
         nread += num_bytes_to_copy;
@@ -1784,5 +1789,4 @@ KResult Ext2FS::prepare_to_unmount() const
     m_inode_cache.clear();
     return KSuccess;
 }
-
 }