KVM: Do not leak memory for duplicate debugfs directories

KVM creates a debugfs directory for each VM in order to store statistics
about the virtual machine.  The directory name is built from the process
pid and a VM fd.  While generally unique, it is possible to keep a
file descriptor alive in a way that causes duplicate directories, which
manifests as these messages:

  [  471.846235] debugfs: Directory '20245-4' with parent 'kvm' already present!

Even though this should not happen in practice, it is more or less
expected in the case of KVM for testcases that call KVM_CREATE_VM and
close the resulting file descriptor repeatedly and in parallel.

When this happens, debugfs_create_dir() returns an error but
kvm_create_vm_debugfs() goes on to allocate stat data structs which are
later leaked.  The slow memory leak was spotted by syzkaller, where it
caused OOM reports.

Since the issue only affects debugfs, do a lookup before calling
debugfs_create_dir, so that the message is downgraded and rate-limited.
W...