qibfs: fix dentry leak

In the Linux kernel, the following vulnerability has been resolved:

qibfs: fix dentry leak

simple_recursive_removal() drops the pinning references to all positives

in subtree. For the cases when its argument has been kept alive by

the pinning alone that's exactly the right thing to do, but here

the argument comes from dcache lookup, that needs to be balanced by

explicit dput().

Fucked-up-by: Al Viro <[email protected]>