dim_calc_stats() may cause uninitialized values to be used

I am reporting the following possible logic bug:

lib/dim/dim.c::dim_calc_stats() doesn't set curr_stats if delta_us == 0,
which would result in using uninitialized values in net_dim_decision()
and rdma_dim_decision().

I don't know anything about this code.  I encountered this only as a
compiler warning compiling an out-of-tree module that had a copy of
dim_calc_stats() inlined for compatibility with old kernels, and I
decided to investigate and report.  There is no compiler warning in
mainline since dim_calc_stats() and net_dim() are in separate C files so
the compiler can't fully analyze it, but it looks like mainline has the
problem also, if the delta_us == 0 condition is possible.

Tony Battersby
Cybernetics