riscv/mp_machdep.c: Flush the TLB after releasing APs

Spurious page faults caused by cached invalid entries may occur when
starting APs and potentially panic the kernel if we're running in
a non-sleepable context.

Fix this avoidable panic by flushing the TLB after the AP is released.

Differential Revision: https://reviews.freebsd.org/D57003
Reviewed by: markj