firmware/efi: Fix a use after bug in efi_mem_reserve_persistent

In the for loop in efi_mem_reserve_persistent(), prsv = rsv->next
use the unmapped rsv. Use the unmapped pages will cause segment
fault.

Fixes: 18df7577 ("efi/memreserve: deal with memreserve entries in unmapped memory")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/efi.c

@@ -927,7 +927,7 @@ int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 	}
 
 	/* first try to find a slot in an existing linked list entry */
-	for (prsv = efi_memreserve_root->next; prsv; prsv = rsv->next) {
+	for (prsv = efi_memreserve_root->next; prsv; ) {
 		rsv = memremap(prsv, sizeof(*rsv), MEMREMAP_WB);
 		index = atomic_fetch_add_unless(&rsv->count, 1, rsv->size);
 		if (index < rsv->size) {
@@ -937,6 +937,7 @@ int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 			memunmap(rsv);
 			return efi_mem_reserve_iomem(addr, size);
 		}
+		prsv = rsv->next;
 		memunmap(rsv);
 	}