nilfs2: integrated block mapping

This adds structures and operations for the block mapping (bmap for
short).  NILFS2 uses direct mappings for short files or B-tree based
mappings for longer files.

Every on-disk data block is held with inodes and managed through this
block mapping.  The nilfs_bmap structure and a set of functions here
provide this capability to the NILFS2 inode.

[[email protected]: remove a bunch of bmap wrapper macros]
[[email protected]: coding-style fixes]
Signed-off-by: Koji Sato <[email protected]>
Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Pekka Enberg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>