staging: lustre: lnet: potential deadlock in lnet

Fixes potential deadlock in LNetMDAttach

Signed-off-by: Quentin Bouget <quentin.bouget.ocre@cea.fr>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8249
Reviewed-on: http://review.whamcloud.com/20676


Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>