LU-12094 ptlrpc: protect obd_import in lprocfs_import_seq_write

LU-12094 ptlrpc: protect obd_import in lprocfs_import_seq_write

The obd_import can disappear so take the cl_sem to protect it.

Change-Id: I36f2c4c606a0470781a692631265938848173f8b
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/35853
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>