xfs: split out inode log item format definition

The log item format definitions are shared with userspace. Split
them out of header files that contain kernel only defintions to make
it simple to shared them.

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>