f2fs: clear SBI_POR_DOING before initing inmem curseg

SBI_POR_DOING can be cleared after recovery is completed, so that
changes made before recovery can be persistent, and subsequent
errors can be recorded into cp/sb.

Signed-off-by: Song Feng <songfeng@oppo.com>
Signed-off-by: Yongpeng Yang <yangyongpeng1@oppo.com>
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>