xfs: split out EFI/EFD log item format definition

The EFI/EFD item format definitions are shared with userspace. Split
the 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>