vmnet: work around skb_frag_t build failure for SLE15-SP2 and openSUSE-15.2

The kernels used in (upcoming) SLE15 SP2 and openSUSE Leap 15.2 are based
on upstream version 5.3 but have a backport of the skb_frag_t conversion to
bio_vec which reached mainline in v5.4-rc1. As we cannot easily check
availability compat_skb_frag_off() (which was also added in v5.4-rc1) and
have to resort to version check, add an exception for SLE15-SP2 and
openSUSE-15.2 kernels.

vmnet-only/userif.c

@@ -78,13 +78,33 @@ static int  VNetUserIfSetUplinkState(VNetPort *port, uint8 linkUp);
 extern unsigned int  vnet_max_qlen;
 
 #if COMPAT_LINUX_VERSION_CHECK_LT(3, 2, 0)
-#   define compat_kmap(page) kmap(page)
-#   define compat_kunmap(page) kunmap(page)
+#   define compat_kmap_frag(frag) kmap((frag)->page)
+#   define compat_kunmap_frag(page) kunmap((frag)->page)
 #else
-#   define compat_kmap(page) kmap((page).p)
-#   define compat_kunmap(page) kunmap((page).p)
+#   define compat_kmap_frag(frag) kmap(skb_frag_page(frag))
+#   define compat_kunmap_frag(frag) kunmap(skb_frag_page(frag))
 #endif
 
+static unsigned int compat_skb_frag_size(const skb_frag_t *frag)
+{
+#if COMPAT_LINUX_VERSION_CHECK_LT(3, 2, 0)
+	return frag->size;
+#else
+	return skb_frag_size(frag);
+#endif
+}
+
+static unsigned int compat_skb_frag_off(const skb_frag_t *frag)
+{
+#if COMPAT_LINUX_VERSION_CHECK_LT(5, 4, 0) && \
+	!(defined(CONFIG_SUSE_VERSION) && CONFIG_SUSE_VERSION == 15 && \
+	  defined(CONFIG_SUSE_PATCHLEVEL) && CONFIG_SUSE_PATCHLEVEL >= 2)
+	return frag->page_offset;
+#else
+	return skb_frag_off(frag);
+#endif
+}
+
 #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 0, 0) && defined(VERIFY_WRITE)
 	#define write_access_ok(addr, size) access_ok(VERIFY_WRITE, addr, size)
 #else
@@ -565,20 +585,21 @@ VNetCsumCopyDatagram(const struct sk_buff *skb,	// IN: skb to copy
    for (frag = skb_shinfo(skb)->frags;
 	frag != skb_shinfo(skb)->frags + skb_shinfo(skb)->nr_frags;
 	frag++) {
-      if (frag->size > 0) {
+      if (compat_skb_frag_size(frag) > 0) {
 	 unsigned int tmpCsum;
 	 const void *vaddr;
 
-	 vaddr = compat_kmap(frag->page);
-	 tmpCsum = csum_and_copy_to_user(vaddr + frag->page_offset,
-					 curr, frag->size, 0, &err);
-	 compat_kunmap(frag->page);
+	 vaddr = compat_kmap_frag(frag);
+	 tmpCsum = csum_and_copy_to_user(vaddr + compat_skb_frag_off(frag),
+					 curr, compat_skb_frag_size(frag), 0,
+					 &err);
+	 compat_kunmap_frag(frag);
 
 	 if (err) {
 	    return err;
 	 }
 	 csum = csum_block_add(csum, tmpCsum, curr - buf);
-	 curr += frag->size;
+	 curr += compat_skb_frag_size(frag);
       }
    }