JFFS2 full summary support (mount time speed up for NAND)

[Ferenc Havasi]

Dear All,

We've finished implementing the summary support for JFFS2. It makes the 
mounting of NAND filesystem much-much faster.

The idea was described before: make some summary information into the 
end of every erase blocks (stored in a RWCOMPAT_DELETE node), so at 
mount time it is enough to read the last few page of eraseblock.

The previous version of our patch supported to generate summary info 
only in image creation time. Now it adds the ability to generate this 
information dimamically, and stores the needed information of every type 
of node.

Certainly the support of it only optional (can be selected by kconfig). 
The usage of it the same as before:

1. generate image as usual
(e.g.: mkfs.jffs2 -r/flash -oimage.jffs2 -n -p -e128KiB)
2. add summary information to the image
(e.g.: sumtool -iimage.jffs2 -oimagesum.jffs2 -e128KiB)
3. write it to the flash as before
(e.g.: nandwrite /dev/mtd0 imagesum.jffs2)
4. compile a new kernel with CONFIG_JFFS2_SUMMARY

David, may I commit it?

Regards,
Ferenc
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