[POWERPC] Change fallocate to match unistd.h on powerpc

Fix the fallocate system call on powerpc to match its unistd.h.

This implies none of these system calls are currently working with the
unistd.h sys call values:
	fallocate
	signalfd
	timerfd
	eventfd
	sync_file_range2

Signed-off-by: Patrick Mansfield <patmans@us.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>