benchtests: Updated json bench-variant attribute

This patch updates json "bench-variant" attribute of "bench-memset.c"
to "default" so that the script "benchtests/scripts/plot_strings.py"
can generate a file "memset_time_default_linear.png".
Without this patch, the script "benchtests/scripts/plot_strings.py"
generates a file "memset_time__linear.png" which has inconsistent form
with "memcpy_time_default_linear.png" and
"memmove_time_default_linear.png".