riscv: Fix another race in pmap_pinit()

Commit c862d5f2a789 ("riscv: Fix a race in pmap_pinit()") did not really
fix the race. Alan writes,

Suppose that N entries in the L1 tables are in use, and we are in the
middle of the memcpy(). Specifically, we have read the zero-filled
(N+1)st entry from the kernel L1 table. Then, we are preempted. Now,
another core/thread does pmap_growkernel(), which fills the (N+1)st
entry. Finally, we return to the original core/thread, and overwrite
the valid entry with the zero that we earlier read.

Try to fix the race properly, by copying kernel L1 entries while holding
the allpmaps lock. To avoid doing unnecessary work while holding this
global lock, copy only the entries that we expect to be valid.

Fixes: c862d5f2a789 ("riscv: Fix a race in pmap_pinit()")
Reported by: alc, jrtc27
Reviewed by: alc
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34267