IB/uverbs: Factor out common idr code

Factor out common code for adding a userspace object to an idr into a
function idr_add_uobj().  This shrinks both the source and object code:

add/remove: 1/0 grow/shrink: 0/6 up/down: 57/-220 (-163)
function                                     old     new   delta
idr_add_uobj                                   -      57     +57
ib_uverbs_create_ah                          543     512     -31
ib_uverbs_create_srq                         662     630     -32
ib_uverbs_reg_mr                             737     699     -38
ib_uverbs_create_cq                          639     600     -39
ib_uverbs_alloc_pd                           485     446     -39
ib_uverbs_create_qp                         1020     979     -41

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