ext2fs: add new APIs needed for fast commits

ext2fs: add new APIs needed for fast commits

This patch adds the following new APIs:

Count the total number of blocks occupied by inode including
intermediate extent tree nodes.
extern errcode_t ext2fs_count_blocks(ext2_filsys fs, ext2_ino_t ino,
                                     struct ext2_inode *inode,
				     blk64_t *ret_count);

Convert the on-disk reputation of an extent to the in-memory
representation.
extern errcode_t ext2fs_decode_extent(struct ext2fs_extent *to, void *from,
                                     int len);

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>