Use SMR to provide safe unlocked lookup for pctries from SMR zones

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.

As future work, it might be nice to add vm_radix algorithms missing from
generic pctrie to the pctrie interface, and then adapt vm_radix to use
pctrie.

Reported by: Attilio
Reviewed by: markj
Sponsored by: Isilon
Differential Revision: https://reviews.freebsd.org/D25781