task_work: add a scheduling point in task_work_run()

It seems commit 4a9d4b02 ("switch fput to task_work_add") re-
introduced the problem addressed in 944be0b2

 ("close_files(): add
scheduling point")

If a server process with a lot of files (say 2 million tcp sockets) is
killed, we can spend a lot of time in task_work_run() and trigger a soft
lockup.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>