simplify compat_sys_timerfd

Just thought this is easier to read.

Acked-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>