libmultipath: lazy tpgs probing

libmultipath: lazy tpgs probing

Provide a "getter" function that can be used to probe tpgs lazily.
This way we don't need to send an RTPG in the pathinfo() call
chain (e.g. in "multipath -u"). With this in place, no "user"
code should access pp->tpgs directly any more.

Moreover, in select_prio(), in the case where the alua checker
was statically configured, rather then calling into the alua
code directly, use get_tpgs(), which does all the proper error
checking, and fall back to const prio if it fails.

Signed-off-by: Martin Wilck <mwilck@suse.com>