compact: workaround for GPL-only symbols on riscv from Linux 6.2

Since Linux 6.2, the implementation of flush_dcache_page on riscv
references GPL-only symbol `PageHuge`, breaking the build of zfs.

This patch uses existing mechanism to override flush_dcache_page,
removing the call to `PageHuge`. According to comments in kernel,
it is only used to do some check against HugeTLB pages, which only
exist in userspace. ZFS uses flush_dcache_page only on kernel pages,
thus this patch will not introduce any behaviour change.

See also: torvalds/linux@d33deda, openzfs/zfs@589f59b

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #14974 
Closes #15627

include/os/linux/kernel/linux/dcache_compat.h

@@ -42,8 +42,8 @@
 /*
  * Starting from Linux 5.13, flush_dcache_page() becomes an inline function
  * and under some configurations, may indirectly referencing GPL-only
- * cpu_feature_keys on powerpc. Override this function when it is detected
- * being GPL-only.
+ * symbols, e.g., cpu_feature_keys on powerpc and PageHuge on riscv.
+ * Override this function when it is detected being GPL-only.
  */
 #if defined __powerpc__ && defined HAVE_FLUSH_DCACHE_PAGE_GPL_ONLY
 #include <linux/simd_powerpc.h>
@@ -53,6 +53,17 @@
 			clear_bit(PG_dcache_clean, &(page)->flags);	\
 	} while (0)
 #endif
+/*
+ * For riscv implementation, the use of PageHuge can be safely removed.
+ * Because it handles pages allocated by HugeTLB, while flush_dcache_page
+ * in zfs module is only called on kernel pages.
+ */
+#if defined __riscv && defined HAVE_FLUSH_DCACHE_PAGE_GPL_ONLY
+#define	flush_dcache_page(page)	do {					\
+		if (test_bit(PG_dcache_clean, &(page)->flags))		\
+			clear_bit(PG_dcache_clean, &(page)->flags);	\
+	} while (0)
+#endif
 
 /*
  * 2.6.30 API change,