libcmfs: early support to namei and extent iteration

This is a big commit, which add quit early support to namei and extent
iteration. Early means currently the code just passes compiling, there
are still some stub functions not implemented yet.

This commit add a new file named libcmfs/namei.c, which privides
functions to find an inode number by a given path name. In order to
support namei mapping, directory block and extent block iteration
functions have to be added in other libcmfs files.

Almost all code are copied and modified from libocfs2, IMHO, current
code is too complexed, cmfs should not be such complicated. I believe in
feature, there are quite a lot extents (data or metadata) iteration
code will be removed from libcmfs.

This commit just passes gcc compiling, there are many bugs and stub
functions to fix.