Discussing with Matthew Wilcox some of his outstanding patches lead me to
this patch (among others).

The preamble in struct sigevent can be expressed independently of the
architecture.

Also use __ARCH_SI_PREAMBLE_SIZE on ia64.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>