f2fs: fix to account preflush command for noflush_merge mode

Previously, we only account preflush command for flush_merge mode,
so for noflush_merge mode, we can not know in-flight preflush
command count, fix it.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -620,7 +620,9 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi, nid_t ino)
 		return 0;
 
 	if (!test_opt(sbi, FLUSH_MERGE)) {
+		atomic_inc(&fcc->issing_flush);
 		ret = submit_flush_wait(sbi, ino);
+		atomic_dec(&fcc->issing_flush);
 		atomic_inc(&fcc->issued_flush);
 		return ret;
 	}