um: virt-pci: Avoid GCC non-NULL warning

GCC gets confused about the return value of get_cpu_var() possibly being NULL, so explicitly test for it before calls to memcpy() and memset(). Avoids warnings like this:

arch/um/drivers/virt-pci.c: In function 'um_pci_send_cmd':
include/linux/fortify-string.h:48:33: warning: argument 1 null where non-null expected [-Wnonnull]

      48 | #define __underlying_memcpy     __builtin_memcpy
         |                                 ^
   include/linux/fortify-string.h:438:9: note: in expansion of macro '__underlying_memcpy'
     438 |         __underlying_##op(p, q, __fortify_size);                        \
         |         ^~~~~~~~~~~~~
   include/linux/fortify-string.h:483:26: note: in expansion of macro '__fortify_memcpy_chk'
     483 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
         |                          ^~~~~~~~~~~~~~~~~~~~
   arch/um/drivers/virt-pci.c:100:9: note: in expansion of macro 'memcpy'
     100 |         memcpy(buf, cmd, cmd_size);
         |         ^~~~~~

While at it, avoid literal "8" and use stored sizeof(buf->data) in memset() and um_pci_send_cmd().

---
I can take this via the hardening tree, since that's where I introduced the warning. :)

---
 arch/um/drivers/virt-pci.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

index acb55b302b14..3ac220dafec4 100644
@@ -97,7 +97,8 @@ static int um_pci_send_cmd(struct um_pci_device *dev,
 	}
 
 	buf = get_cpu_var(um_pci_msg_bufs);
-	memcpy(buf, cmd, cmd_size);
+	if (buf)
+		memcpy(buf, cmd, cmd_size);
 
 	if (posted) {
 		u8 *ncmd = kmalloc(cmd_size + extra_size, GFP_ATOMIC);

@@ -182,6 +183,7 @@ static unsigned long um_pci_cfgspace_read(void *priv, unsigned int offset,
 	struct um_pci_message_buffer *buf;
 	u8 *data;
 	unsigned long ret = ULONG_MAX;
+	size_t bytes = sizeof(buf->data);
 
 	if (!dev)
 		return ULONG_MAX;

@@ -189,7 +191,8 @@ static unsigned long um_pci_cfgspace_read(void *priv, unsigned int offset,
 	buf = get_cpu_var(um_pci_msg_bufs);
 	data = buf->data;
 
-	memset(buf->data, 0xff, sizeof(buf->data));
+	if (buf)
+		memset(data, 0xff, bytes);
 
 	switch (size) {
 	case 1:

@@ -204,7 +207,7 @@ static unsigned long um_pci_cfgspace_read(void *priv, unsigned int offset,
 		goto out;
 	}
 
-	if (um_pci_send_cmd(dev, &hdr, sizeof(hdr), NULL, 0, data, 8))
+	if (um_pci_send_cmd(dev, &hdr, sizeof(hdr), NULL, 0, data, bytes))
 		goto out;
 
 	switch (size) {

2.34.1