Pass full strings to DTrace probes

Whenever string is passed as an argument to a DTrace probe, its length
should be properly computed. Until now in order to get length of the
input buffer size_of(char *) was used - which evalutes to 4 or 8
(depending on the architecture). To get a proper length,
size_of(DTRACE_CHARBUF_NAME(buffer_name)) should be used.