UBUNTU: SAUCE: shiftfs: fix buggy unlink logic

UBUNTU: SAUCE: shiftfs: fix buggy unlink logic

BugLink: https://bugs.launchpad.net/bugs/1841977
The way we messed with setting i_nlink was brittle and wrong. We used to
set the i_nlink of the shiftfs dentry to be deleted to the i_nlink count
of the underlay dentry of the directory it resided in which makes no
sense whatsoever. We also missed drop_nlink() which is crucial since
i_nlink affects whether a dentry is cleaned up on dput().
With this I cannot reproduce the bug anymore where shiftfs misleads zfs
into believing that a deleted file can not be removed from disk because
it is still referenced.

Fixes: commit 87011da41961 ("shiftfs: rework and extend")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>