ARCv3: fix valid pfns counting

Fix min_low_pfn counting. For ARCv3 explicit use of __pa() modifies address
which cause to bad min_low_pfn value. For configurations where
CONFIG_LINUX_RAM_BASE is not equal to PAGE_OFFSET we should consider
CONFIG_LINUX_RAM_BASE as physical address.

This patch fixes min_low_pfn value counting. This variable is used to check
virt address by virt_addr_valid() macro to determine that the address resides
in the kernel "direct mapping area".

Without this patch some unexpected kernel crashes can be observed, for
example in __check_object_size() function because of incorrect
check of virt_addr_valid() macro, virt_to_page() macro can be called with
virt address from VMALLOC area and return bad pointer to page struct object.

This change is actual for ARCv3 not affect behavior of ARCv2.

Signed-off-by: Pavel Kozlov <pavel.kozlov@synopsys.com>

Author: pavelvkozlov

arch/arc/mm/init.c

@@ -102,7 +102,7 @@ void __init setup_arch_memory(void)
 	setup_initial_init_mm(_text, _etext, _edata, _end);
 
 	/* first page of system - kernel .vector starts here */
-	min_low_pfn = virt_to_pfn(CONFIG_LINUX_RAM_BASE);
+	min_low_pfn = PFN_DOWN(low_mem_start);
 
 	/* Last usable page of low mem */
 	max_low_pfn = max_pfn = PFN_DOWN(low_mem_start + low_mem_sz);