riscv: Sync efi page table's kernel mappings before switching

In the Linux kernel, the following vulnerability has been resolved:

riscv: Sync efi page table's kernel mappings before switching

The EFI page table is initially created as a copy of the kernel page table.

With VMAP_STACK enabled, kernel stacks are allocated in the vmalloc area:

if the stack is allocated in a new PGD (one that was not present at the

moment of the efi page table creation or not synced in a previous vmalloc

fault), the kernel will take a trap when switching to the efi page table

when the vmalloc kernel stack is accessed, resulting in a kernel panic.

Fix that by updating the efi kernel mappings before switching to the efi

page table.