xfs: split in-core and on-disk inode log item fields

Add a new ili_fields member to the inode log item to isolate the in-memory
flags from the ones that actually go to the log.  This will allow tracking
timestamp-only updates for fdatasync and O_DSYNC in the next patch and
prepares for divorcing the on-disk log format from the in-memory log item
a little further down the road.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>