Increase the max allowed size of the microcode update blob for x86.

Newer CPUs (SkyLakes) have updates of 100K size, which is bigger than
current limit 32K. Increase it to 4M but leave the check around to
prevent kernel memory allocator abuse.  Some time ago, the memory for
update was allocated by contigmalloc(9), and it was reasonable to be
conservative as much as possible.  Since all uses of contigmalloc(9)
appear to be either misunderstanding or too cautious, and were
removed, provide more slack than strictly neccessary.

Submitted by:	Oliver Pinter
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D8486

sys/dev/cpuctl/cpuctl.c

@@ -63,7 +63,7 @@ static d_ioctl_t cpuctl_ioctl;
 # define	DPRINTF(...)
 #endif
 
-#define	UCODE_SIZE_MAX	(32 * 1024)
+#define	UCODE_SIZE_MAX	(4 * 1024 * 1024)
 
 static int cpuctl_do_msr(int cpu, cpuctl_msr_args_t *data, u_long cmd,
     struct thread *td);