Adapt r358130, for the almost identical vm_radix, to the pctrie subsystem.
Like that change, the tree is kept correct for readers with store barriers
and careful ordering. Existing locks serialize writers.
Add a PCTRIE_DEFINE_SMR() wrapper that takes an additional smr_t parameter
and instantiates a FOO_PCTRIE_LOOKUP_UNLOCKED() function, in addition to the
usual definitions created by PCTRIE_DEFINE().
Interface consumers will be introduced in later commits.