ext4: drop legacy pre-1970 encoding workaround

Originally, support for expanded timestamps had a bug in that pre-1970
times were erroneously encoded as being in the the 24th century.  This
was fixed in commit a4dad1ae ("ext4: Fix handling of extended
tv_sec") which landed in 4.4.  Starting with 4.4, pre-1970 timestamps
were correctly encoded, but for backwards compatibility those
incorrectly encoded timestamps were mapped back to the pre-1970 dates.

Given that backwards compatibility workaround has been around for 4
years, and given that running e2fsck from e2fsprogs 1.43.2 and later
will offer to fix these timestamps (which has been released for 3
years), it's past time to drop the legacy workaround from the kernel.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -828,21 +828,8 @@ static inline __le32 ext4_encode_extra_time(struct timespec64 *time)
 static inline void ext4_decode_extra_time(struct timespec64 *time,
 					  __le32 extra)
 {
-	if (unlikely(extra & cpu_to_le32(EXT4_EPOCH_MASK))) {
-
-#if 1
-		/* Handle legacy encoding of pre-1970 dates with epoch
-		 * bits 1,1. (This backwards compatibility may be removed
-		 * at the discretion of the ext4 developers.)
-		 */
-		u64 extra_bits = le32_to_cpu(extra) & EXT4_EPOCH_MASK;
-		if (extra_bits == 3 && ((time->tv_sec) & 0x80000000) != 0)
-			extra_bits = 0;
-		time->tv_sec += extra_bits << 32;
-#else
+	if (unlikely(extra & cpu_to_le32(EXT4_EPOCH_MASK)))
 		time->tv_sec += (u64)(le32_to_cpu(extra) & EXT4_EPOCH_MASK) << 32;
-#endif
-	}
 	time->tv_nsec = (le32_to_cpu(extra) & EXT4_NSEC_MASK) >> EXT4_EPOCH_BITS;
 }