tracehook: kerneldoc fix

My last change to tracehook.h made it confuse the kerneldoc parser.
Move the #define's before the comment so it's happy again.

Signed-off-by: Roland McGrath <roland@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>