task_work: minor cleanups

Trivial.  Remove the unnecessary "work = NULL" initialization and turn
read_barrier_depends() into smp_read_barrier_depends() in
task_work_cancel().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>