fs: add timespec64_truncate()

As vfs moves to using struct timespec64 to represent times,
update the argument to timespec_truncate() to use
struct timespec64. Also change the name of the function.
The rest of the implementation logic is the same.

Move this to fs/inode.c instead of kernel/time/time.c as all the
users of this api are filesystems.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -2110,6 +2110,30 @@ void inode_nohighmem(struct inode *inode)
 }
 EXPORT_SYMBOL(inode_nohighmem);
 
+/**
+ * timespec64_trunc - Truncate timespec64 to a granularity
+ * @t: Timespec64
+ * @gran: Granularity in ns.
+ *
+ * Truncate a timespec64 to a granularity. Always rounds down. gran must
+ * not be 0 nor greater than a second (NSEC_PER_SEC, or 10^9 ns).
+ */
+struct timespec64 timespec64_trunc(struct timespec64 t, unsigned gran)
+{
+	/* Avoid division in the common cases 1 ns and 1 s. */
+	if (gran == 1) {
+		/* nothing */
+	} else if (gran == NSEC_PER_SEC) {
+		t.tv_nsec = 0;
+	} else if (gran > 1 && gran < NSEC_PER_SEC) {
+		t.tv_nsec -= t.tv_nsec % gran;
+	} else {
+		WARN(1, "illegal file time granularity: %u", gran);
+	}
+	return t;
+}
+EXPORT_SYMBOL(timespec64_trunc);
+
 /**
  * current_time - Return FS time
  * @inode: inode.

include/linux/fs.h

@@ -1476,6 +1476,7 @@ static inline void i_gid_write(struct inode *inode, gid_t gid)
 	inode->i_gid = make_kgid(inode->i_sb->s_user_ns, gid);
 }
 
+extern struct timespec64 timespec64_trunc(struct timespec64 t, unsigned gran);
 extern struct timespec current_time(struct inode *inode);
 
 /*