The vzalloc()'ed field physshadow is leaked on module unload.

This patch adds vfree after the sibling page shadow is freed.
Reported-by: Dean Luick <dean.luick@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>