Use sscanf() for parsing tag values.

Neither strtoll() nor strtoull() can parse the %llx formatted tag
values coming from the kernel.  We know the lowest 32 bits will never
be set, so fast-path the 3-character case as 0x0.

Bug: 17365163
Change-Id: I238bbd2830c9335e7ab7a53362d6e12b46e0bcb3