eCryptfs: Invalidate dcache entries when lower i_nlink is zero

commit 5556e7e6 upstream.

Consider eCryptfs dcache entries to be stale when the corresponding
lower inode's i_nlink count is zero. This solves a problem caused by the
lower inode being directly modified, without going through the eCryptfs
mount, leaving stale eCryptfs dentries cached and the eCryptfs inode's
i_nlink count not being cleared.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Reported-by: Richard Weinberger <richard@nod.at>
[bwh: Backported to 3.2:
 - Test d_revalidate pointer directly rather than a DCACHE_OP flag
 - Open-code d_inode()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>