ecryptfs: Set s_time_gran to get correct time granularity

Set the eCryptfs superblock time granularity, using the lower
filesystem's s_time_gran value, to prevent unnecessary inode timestamp
truncation to the granularity of a full second.

The use of utimensat(2) to set a timestamp with nanosecond precision
would trigger this bug. That occurred when using the following utilities
to update timestamps of a file:
  * cp -p: copy a file and preserve its atime and mtime
  * touch -r: touch a file and use a reference file's timestamps

Closes: https://bugs.launchpad.net/ecryptfs/+bug/1890486
Signed-off-by: Frank Hsiao 蕭法宣 <frankhsiao@qnap.com>
[tyhicks: Partially rewrite the commit message]
Signed-off-by: Tyler Hicks <code@tyhicks.com>

Author: Frank Hsiao 蕭法宣

fs/ecryptfs/main.c

@@ -531,6 +531,7 @@ static int ecryptfs_get_tree(struct fs_context *fc)
 	s->s_blocksize = path.dentry->d_sb->s_blocksize;
 	s->s_magic = ECRYPTFS_SUPER_MAGIC;
 	s->s_stack_depth = path.dentry->d_sb->s_stack_depth + 1;
+	s->s_time_gran = path.dentry->d_sb->s_time_gran;
 
 	rc = -EINVAL;
 	if (s->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {