UPSTREAM: f2fs: introduce migration_window_granularity

We can control the scanning window granularity for GC migration. For
more frequent scanning and GC on zoned devices, we need a fine grained
control knob for it.

Change-Id: Iee24b68453c8cc4b9192abad691da6d6522cd6be
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 8c890c4c)