IB/iser: Make a couple of functions static

Make iser_conn_release() and iser_start_rdma_unaligned_sg() static,
since they are only used in the .c file where they are defined.  In
addition to being a cleanup, this even shrinks the generated code by
allowing the single call of iser_start_rdma_unaligned_sg() to be
inlined into its callsite.  On x86_64:

add/remove: 0/1 grow/shrink: 1/0 up/down: 466/-533 (-67)
function                                     old     new   delta
iser_reg_rdma_mem                           1518    1984    +466
iser_start_rdma_unaligned_sg                 533       -    -533

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