f2fs: fix gc mode when gc_urgent_high_remaining is 1

Under the current logic, when gc_urgent_high_remaining is set to 1,
the mode will be switched to normal at the beginning, instead of
running in gc_urgent mode.

Let's switch the gc mode back to normal when the gc ends.

Fixes: 265576181b4a ("f2fs: remove gc_urgent_high_limited for cleanup")
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>