efivarfs: update ctime when mtime changes on a write

POSIX says:

"Upon successful completion, where nbyte is greater than 0, write()
 shall mark for update the last data modification and last file status
 change timestamps of the file..."

Add the missing ctime update.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Message-Id: <20230705190309.579783-3-jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>