LU-7430 mdt: better handle MDT recovery error path

LU-7430 mdt: better handle MDT recovery error path

This fix ensures the lu_target structure is still valid before
updating export related data in tgt_client_free().

In case of recovery error, it may happen that lu_target and mdt_device
structures are freed before exports are destroyed.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I0559d42f40d4ecc9078717aa07b988be56ce3bc5
Reviewed-on: http://review.whamcloud.com/17424
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>