writeback: fixups for !dirty_writeback_centisecs

writeback: fixups for !dirty_writeback_centisecs

commit 6423104b6a1e6f0c18be60e8c33f02d263331d5e upstream.

Commit 69b62d01 fixed up most of the places where we would enter
busy schedule() spins when disabling the periodic background
writeback. This fixes up the sb timer so that it doesn't get
hammered on with the delay disabled, and ensures that it gets
rearmed if needed when /proc/sys/vm/dirty_writeback_centisecs
gets modified.

bdi_forker_task() also needs to check for !dirty_writeback_centisecs
and use schedule() appropriately, fix that up too.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Tested-by: Xavier Roche <roche@httrack.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>