Convert init_sync_kiocb() from a nasty macro into a nice C function. The
struct assignment trick takes care of zeroing all unmentioned fields.
Shrinks fs/read_write.o's .text from 9857 bytes to 9714.
Also demacroize is_sync_kiocb() and aio_ring_avail(). The latter fixes an
arg-referenced-multiple-times hand grenade.
Cc: Junxiao Bi <junxiao.bi@oracle.com> Cc: Mark Fasheh <mfasheh@suse.com> Acked-by: Jeff Moyer <jmoyer@redhat.com> Cc: Joel Becker <jlbec@evilplan.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>