Optimize: increase the default size of the vma map hash table

Optimize: increase the default size of the vma map hash table

Hash conflicts is severe when the vma tracker keeps track of many
processes given the current hash table size of mere 16 buckets.
Increased it to 256 buckets by default and also make it tunable
via the macro __STP_TF_HASH_BITS.

In our test, this significantly reduces hash conflicts when
stap/staprun's -x PID option is not specified.