It is entirely possible (though rare) that jiffies half-wraps around, while a
dentry/inode remains in the cache. This could mean that the dentry/inode is
not invalidated for another half wraparound-time.
To get around this problem, use 64-bit jiffies. The only problem with this is
that dentry->d_time is 32 bits on 32-bit archs. So use d_fsdata as the high
32 bits. This is an ugly hack, but far simpler, than having to allocate
private data just for this purpose.
Since 64-bit jiffies can be assumed never to wrap around, simple comparison
can be used, and a zero time value can represent "invalid".