[RFC 5/5] inode reservation v0.1 (next step working)

From the benchmark and the experimental inores patch on ext4, it can be

found that inode reservation on ext4 is a good idea to be tried. 

One of the original idea of inode reservation is NOT modifying on-disk
format. Current magic inode can make it, but use a magic inodes list to
link each reserved inode block together is not a good idea. Indeed, this
is a performance killer.

1, Therefor, group descriptor has to be modified to add a new member ---
this new member records the lastest reserved inode in inode table of
each block group.

2, Use rest room in magic inode (sizeof(ext4_inode) -
sizeof(ext4_magic_inode)) to record other reserved inode blocks. This
method can reduce number of magic inodes, which can minimize I/O for
magic inodes.

3, Use magic inode cache. Most of the magic inode accessings are reading
(not writing), therefore, caching can help to reduce most of the read
I/O for magic inode.

4, Modify mke2fs to support new on-disk layout for inode reservation.


Coly