Correct refcount_add in dmu_zfetch

refcount_add_many(foo,N) is not the same as
for (i=0; i < N; i++) { refcount_add(foo); }

Unfortunately, this is only actually true with debug kernels and
reference_tracking_enable=1.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12589
Closes #12602