The per-metadata-type ocfs2_journal_access_*() functions hook up jbd2
commit triggers and allow us to compute metadata ecc right before the
buffers are written out.  This commit provides ecc for inodes, extent
blocks, group descriptors, and quota blocks.  It is not safe to use
extened attributes and metaecc at the same time yet.

The ocfs2_extent_tree and ocfs2_path abstractions in alloc.c both hide
the type of block at their root.  Before, it didn't matter, but now the
root block must use the appropriate ocfs2_journal_access_*() function.
To keep this abstract, the structures now have a pointer to the matching
journal_access function and a wrapper call to call it.

A few places use naked ocfs2_write_block() calls instead of adding the
blocks to the journal.  We make sure to calculate their checksum and ecc
before the write.

Since we pass around the journal_access functions.  Let's typedef them
in ocfs2.h.
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>