nilfs2: use time64_t internally

The superblock and segment timestamps are used only internally in nilfs2
and can be read out using sysfs.

Since we are using the old 'get_seconds()' interface and store the data
as timestamps, the behavior differs slightly between 64-bit and 32-bit
kernels, the latter will show incorrect timestamps after 2038 in sysfs,
and presumably fail completely in 2106 as comparisons go wrong.

This changes nilfs2 to use time64_t with ktime_get_real_seconds() to
handle timestamps, making the behavior consistent and correct on both
32-bit and 64-bit machines.

The on-disk format already uses 64-bit timestamps, so nothing changes
there.

Link: http://lkml.kernel.org/r/20180122211050.1286441-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/the_nilfs.h

@@ -116,7 +116,7 @@ struct the_nilfs {
 	 */
 	struct buffer_head     *ns_sbh[2];
 	struct nilfs_super_block *ns_sbp[2];
-	time_t			ns_sbwtime;
+	time64_t		ns_sbwtime;
 	unsigned int		ns_sbwcount;
 	unsigned int		ns_sbsize;
 	unsigned int		ns_mount_state;
@@ -131,8 +131,8 @@ struct the_nilfs {
 	__u64			ns_nextnum;
 	unsigned long		ns_pseg_offset;
 	__u64			ns_cno;
-	time_t			ns_ctime;
-	time_t			ns_nongc_ctime;
+	time64_t		ns_ctime;
+	time64_t		ns_nongc_ctime;
 	atomic_t		ns_ndirtyblks;
 
 	/*
@@ -267,7 +267,7 @@ struct nilfs_root {
 
 static inline int nilfs_sb_need_update(struct the_nilfs *nilfs)
 {
-	u64 t = get_seconds();
+	u64 t = ktime_get_real_seconds();
 
 	return t < nilfs->ns_sbwtime ||
 		t > nilfs->ns_sbwtime + nilfs->ns_sb_update_freq;