f2fs: add gc_boost_gc_multiple sysfs node

f2fs: add gc_boost_gc_multiple sysfs node

[ Upstream commit 1d4c5dbba1a53aeaf2c6cc84e7ba94c436d18852 ]

Add a sysfs knob to set a multiplier for the background GC migration
window when F2FS Garbage Collection is boosted.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: e462fc48ceb8 ("f2fs: maintain one time GC mode is enabled during whole zoned GC cycle")
Signed-off-by: Sasha Levin <sashal@kernel.org>