UPSTREAM: debugfs: split end_creating() into success and failure cases

... and don't bother with dput(dentry) in the former and with
dget(dentry) preceding all its calls.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Bug: 31856701
Change-Id: I4f4aa36421f5b8beed9ed3a20c9c8ba1c511e98a
(cherry picked from commit 5233e31191af661389a4f5b060873bfcb155c828)
Signed-off-by: Paul Lawrence <paullawrence@google.com>