SUNRPC: NULL utsname dereference on NFS umount during namespace cleanup

SUNRPC: NULL utsname dereference on NFS umount during namespace cleanup

commit 03a9a42a1a7e5b3e7919ddfacc1d1cc81882a955 upstream.

Fix an Oopsable condition when nsm_mon_unmon is called as part of the
namespace cleanup, which now apparently happens after the utsname
has been freed.

Link: http://lkml.kernel.org/r/20150125220604.090121ae@neptune.home
Reported-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>