f2fs: check curseg space before foreground GC

When we are closing to trigger foreground GC, if there are only a few
of dirty metas, we can log these dirty metas in left space of opened
segments instead of triggering foreground GC.

With this patch, total count of foreground GC triggered by
test/generic/* of fstest suit reduce from 254 to 184.

So let's do the check before foreground GC anyway.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>