Add bankshot2_check_extents

Basically copied from do_xip_mapping_read(), filemap_xip.c
Add bankshot2_xip.c. It provides the cache file memory address an pfn
to request from upper layer ioctl request.
Add bankshot2_find_data_block

It finds the offset to the block represented by the given inode's file
relative block number.
Add bankshot2_get_pfn