LU-1092 ptlrpc: take export refcount during connect

LU-1092 ptlrpc: take export refcount during connect

In the process of (re)connect,  a refcount of export should be taken,
otherwise disconnect of this export may be called, and it will put
the last refcount of this export and make access to this export
invalid.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: Iaf27e842ed516b8968c90bfce396609e39f52c85
Reviewed-on: http://review.whamcloud.com/2345
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>