dtrace: avoid gcc9 Walloca-larger-than

gcc9 grew a new warning for unbounded allocas, such as the one in
dt_options_load. Remove both uses of alloca in dt_options.c.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22880

(cherry picked from commit fa19b250bdb7e463b58a8e7628ebea09a44197f3)