IPoIB: In unicast_arp_send(), only free newly-created paths

If path_rec_start() returns error, call path_free() only if the path
was newly-created.  If we free an existing path whose valid flag was zero,
(but do not detach it from the list) we cause corruption of the
path list (of which it is a member), and get a kernel crash.

The simplest solution is to not free an existing path -- just leave it
in the list as-is (i.e., with its valid flag cleared).

Thanks to Yossi Etigin of Voltaire for identifying the problem flow
which caused the kernel crash.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Moni Shua <monis@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>