proc_kill_inodes() walks the s_files list, playing with ->f_dentry.

But there is a window in which __fput() will leave a file on that list with a
null f_dentry and f_vfsmnt.

I'm not sure it was ever confirmed that this fixed the reported oops, but it
seems much better to set those fields to null _after_ removing the filp from
the list.

(Actually, there's no need to null those pointers out at all.  But whatever;
it caught a bug).