[IPoIB] Rename IPoIB's path_lookup() to avoid name clashes

Rename IPoIB driver's path_lookup() to ipoib_path_lookup() to avoid a
clashes with the kernel global path_lookup().  We don't hit this with
the current kernel source, but some external patches seem to trigger
this, and it's cleaner to avoid clashing with global names anyway.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
refs/heads/for-linus