IPoIB: Fix crash in path_rec_completion()

Fix a crash in path_rec_completion() during an SM up/down loop.  If
more than one path record request is issued, the first completion
releases path->done, allowing ipoib_flush_paths() to free the path,
and thus corrupting it for the second completion.

Commit ee1e2c82

 ("IPoIB: Refresh paths instead of flushing them on SM
change events") added the field path->valid and changed the test "if
(!path)" to "if (!path || !path->valid)".  This change made it
possible for a path with an outstanding query to pass the test and
issue another query on the same path.  Having two queries on the same
path leads to a crash.

This fixes <https://bugs.openfabrics.org/show_bug.cgi?id=1325>.

Signed-off-by: Yossi Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>