IB/uverbs: use stack variable 'base' in ib_uverbs_add_one

This change is not useful by itself, but sets us up for a future change
that allows us to support more than IB_UVERBS_MAX_DEVICES in a system.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>