f2fs: use aggressive GC policy during f2fs_disable_checkpoint()

Let's enable GC_URGENT_HIGH mode during f2fs_disable_checkpoint(),
so that we can use SSR allocator for GCed data/node persistence,
it can improve the performance due to it avoiding migration of
data/node locates in selected target segment of SSR allocator.

Signed-off-by: Chao Yu <chao.yu@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>