[victoriafalls] Move the block to lba translation to a small wrapper.

Abstract this away into a thin wrapper over the denali client such that
it can be called from other helper classes.

sys/victoriafalls/fs/ext2/ext2_block_reader.cpp

@@ -0,0 +1,32 @@
+#include "fs/ext2/ext2_block_reader.h"
+
+glcr::ErrorOr<Ext2BlockReader> Ext2BlockReader::Init(
+    ScopedDenaliClient&& denali) {
+  // Read 1024 bytes from 1024 offset.
+  // FIXME: Don't assume 512 byte sectors somehow.
+  ASSIGN_OR_RETURN(MappedMemoryRegion superblock, denali.ReadSectors(2, 2));
+  return Ext2BlockReader(glcr::Move(denali), superblock);
+}
+
+Superblock* Ext2BlockReader::GetSuperblock() {
+  return reinterpret_cast<Superblock*>(super_block_region_.vaddr());
+}
+
+uint64_t Ext2BlockReader::SectorsPerBlock() {
+  return 1 << (GetSuperblock()->log_block_size + 1);
+}
+
+glcr::ErrorOr<MappedMemoryRegion> Ext2BlockReader::ReadBlock(
+    uint64_t block_number) {
+  return denali_.ReadSectors(block_number * SectorsPerBlock(),
+                             SectorsPerBlock());
+}
+glcr::ErrorOr<MappedMemoryRegion> Ext2BlockReader::ReadBlocks(
+    uint64_t block_number, uint64_t num_blocks) {
+  return denali_.ReadSectors(block_number * SectorsPerBlock(),
+                             num_blocks * SectorsPerBlock());
+}
+
+Ext2BlockReader::Ext2BlockReader(ScopedDenaliClient&& denali,
+                                 MappedMemoryRegion super_block)
+    : denali_(glcr::Move(denali)), super_block_region_(super_block) {}