Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

include/os/freebsd/linux/compiler.h

@@ -67,6 +67,7 @@
 #define	__always_inline			inline
 #define	noinline			__noinline
 #define	____cacheline_aligned		__aligned(CACHE_LINE_SIZE)
+#define	fallthrough			__attribute__((__fallthrough__))
 
 #if !defined(_KERNEL) && !defined(_STANDALONE)
 #define	likely(x)			__builtin_expect(!!(x), 1)

include/os/linux/kernel/linux/compiler_compat.h

@@ -28,6 +28,14 @@
 
 #include <linux/compiler.h>
 
+#if !defined(fallthrough)
+#if defined(HAVE_IMPLICIT_FALLTHROUGH)
+#define	fallthrough		__attribute__((__fallthrough__))
+#else
+#define	fallthrough		((void)0)
+#endif
+#endif
+
 #if !defined(READ_ONCE)
 #define	READ_ONCE(x)		ACCESS_ONCE(x)
 #endif

include/os/linux/zfs/sys/zfs_context_os.h

@@ -25,6 +25,7 @@
 
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
+#include <linux/compiler_compat.h>
 #include <linux/module.h>
 
 #if THREAD_SIZE >= 16384