LU-13116 ptlrpc: return error for conn with NULL export

LU-13116 ptlrpc: return error for conn with NULL export

In import_select_connection(), instead of asserting when export on
chosen connection is NULL, return an error.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I60dcb26693f2ba177d612ded7efff9721febf601
Reviewed-on: https://review.whamcloud.com/37251
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>