Add bankshot2_find_cache_inode

Which finds the corresponding cache inode in bankshot2, or allocate
a new cache inode if not found.
Set pi->backup_ino
When cache inode is not set in cache data, search for existing
matching cache inodes first.
bankshot2_xip_file_write
bankshot2_xip_file_read part1
bankshot2_xip_file_read part2
xip_file_read fix
Pass xmem to io functions