Release the pmap lock when the reference/change-bit compare-and-swap
fails. The retry path acquires the lock again, so retaining it can
recursively acquire a non-recursive mutex after a concurrent update.
Fixes: 65bbba25d214 ("powerpc64: Implement Radix MMU for POWER9 CPUs")
MFC after: 2 weeks
Sponsored by: FreeBSD Foundation, Reliable Computer Systems Lab