xfs: separate icreate log format definitions from xfs_icreate_item.h

The on disk log format definitions for the icreate log item are
intertwined with the kernel-only in-memory log item definitions.
Separate the log format definitions out into their own header file
so they can easily be shared with userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>