staging: lustre: llite: Adding timed wait in ll_umount_begin

staging: lustre: llite: Adding timed wait in ll_umount_begin

There exists timing race between umount and other
thread which will increment the reference count on
mnt e.g. getattr. If umount thread lose the race
then umount fails with EBUSY error. To avoid this
timed wait is added so that umount thread will wait
for user to decrement the mnt reference count.

Signed-off-by: Rahul Deshmukh <rahul.deshmukh@seagate.com>
Signed-off-by: Lokesh Nagappa Jaliminche <lokesh.jaliminche@seagate.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-1882
Seagate-bug-id: MRP-1192
Reviewed-on: http://review.whamcloud.com/20061
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
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>