staging/lustre/ldlm: fix NULL pointer dereference

Pointer '*exp' returned from call to function 'class_conn2export'
at line 523 may be NULL and may be dereferenced at line 543.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/9323
ntel-bug-id: https://jira.hpdd.intel.com/browse/LU-4629


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>