libmulitpath: cleanup uid_fallback code

libmulitpath: cleanup uid_fallback code

Instead of always calling uid_fallback() if the configured method to get
the uid failed, get_uid now checks if the path supports fallbacks and if
all the retriggers have occurred. If so, it calls uid_fallback(), which
just attempts to get the uid using the appropriate fallback method. None
of these changes should make the code function any differently.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>