powerpc/64: Fix strncpy() related build failures with GCC 8.1

GCC 8.1 warns about possible string truncation:

arch/powerpc/kernel/nvram_64.c:1042:2: error: 'strncpy' specified
bound 12 equals destination size [-Werror=stringop-truncation]
strncpy(new_part->header.name, name, 12);

arch/powerpc/platforms/ps3/repository.c:106:2: error: 'strncpy'
output truncated before terminating nul copying 8 bytes from a
string of the same length [-Werror=stringop-truncation]
strncpy((char *)&n, text, 8);

Fix it by using memcpy(). To make that safe we need to ensure the
destination is pre-zeroed. Use kzalloc() in the nvram code and
initialise the u64 to zero in the ps3 code.

Signed-off-by: Christophe Leroy <[e-mail]>
[mpe: Use kzalloc() in the nvram code, flesh out change log]
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/kernel/nvram_64.c

···
 		return -ENOSPC;
 	
 	/* Create our OS partition */
-	new_part = kmalloc(sizeof(*new_part), GFP_KERNEL);
+	new_part = kzalloc(sizeof(*new_part), GFP_KERNEL);
 	if (!new_part) {
 		pr_err("%s: kmalloc failed\n", __func__);
 		return -ENOMEM;
···
 	new_part->index = free_part->index;
 	new_part->header.signature = sig;
 	new_part->header.length = size;
-	strncpy(new_part->header.name, name, 12);
+	memcpy(new_part->header.name, name, strnlen(name, sizeof(new_part->header.name)));
 	new_part->header.checksum = nvram_checksum(&new_part->header);
 
 	rc = nvram_write_header(new_part);

arch/powerpc/platforms/ps3/repository.c

···
 
 static u64 make_field(const char *text, u64 index)
 {
-	u64 n;
+	u64 n = 0;
 
-	strncpy((char *)&n, text, 8);
+	memcpy((char *)&n, text, strnlen(text, sizeof(n)));
 	return n + index;
 }