[PATCH]fs/jffs2/wbuf.c: add compatibility support for OOB data block

[赵 豆豆]

Hi, 

Current jffs2_check_oob_empty() assumes that only OOB of first
page has data content -- the clean marker. OOB of other pages should
be all 0xFF.
But if we decide to store other kind of meta-data in OOB of the
second,third or forth page, this will cause compatibility issue.
For example, if we store some data in OOB of second page in the
future, the "old" JFFS2 will recognize all the erase blocks in
the "new" JFFS2 image as ALL_DIRTY.
So this patch solve this compatibility problem by extending existent
node type compat_bits to OOB area. In particular, I think
JFFS2_FEATURE_RWCOMPAT_COPY has no meaning for OOB, so it's
handled the same way as JFFS2_FEATURE_RWCOMPAT_DELETE.

Your comments are welcome!

Thanks,
Forrest

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