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

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