ext4: fix fencepost error in lazytime optimization

ext4: fix fencepost error in lazytime optimization

commit 0f0ff9a9f3fa2ec6f427603fd521d5f3a0b076d1 upstream.

Commit 8f4d8558391: "ext4: fix lazytime optimization" was not a
complete fix.  In the case where the inode number is a multiple of 16,
and we could still end up updating an inode with dirty timestamps
written to the wrong inode on disk.  Oops.

This can be easily reproduced by using generic/005 with a file system
with metadata_csum and lazytime enabled.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>