IPoIB: reinitialize path struct's completion for every query

It's possible that IPoIB will issue multiple SA queries for the same
path struct.  Therefore the struct's completion needs to be
initialized for each query rather than only once when the struct is
allocated, or else we might not wait long enough for later queries to
finish and free the path struct too soon.

Signed-off-by: Roland Dreier <rolandd@cisco.com>