FAT: use io_schedule_timeout() instead of congestion_wait()

congestion_wait() in this context is just a sleep - block devices do not
support congestion signalling any more.

The goal for this wait, which was introduced in commit ae78bf9c


("[PATCH] add -o flush for fat") is to wait for any recently written
data to get to storage.  We currently have no direct mechanism to do
this, so a simple wait that behaves identically to the current
congestion_wait() is the best we can do.

This is a step towards removing congestion_wait()

Link: https://lkml.kernel.org/r/163936544519.22433.13400436295732112065@noble.neil.brown.name
Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>