nvmecontrol wdc: Don't pass a bogus pointer to free().

wdc_get_dui_log_size allocates a buffer and then advances the
returned pointer. Passing this advanced pointer to free() is UB,
so save the original pointer to pass to free() instead.

Reviewed by: imp
Reported by: GCC 12 -Wfree-nonheap-object
Differential Revision: https://reviews.freebsd.org/D36827