cpuset: increase userland maximum size to 1024

Hardware with more than 256 CPU cores is now available and will become
increasingly common.  Bump CPU_MAXSIZE (used for userland cpuset_t
sizing) to 1024 to define the ABI for FreeBSD 14.

This change is reapplied after a change to decouple cpuset from bhyve:
commit e17eca3 ("vmm: Avoid embedding cpuset_t ioctl ABIs").

PR:		269572, 271213 [exp-run]
Reviewed by:	mjg, jhb
Relnotes:	yes
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39941

(cherry picked from commit 76887e8)

sys/sys/_cpuset.h

@@ -40,7 +40,7 @@
 #define	CPU_SETSIZE	MAXCPU
 #endif
 
-#define	CPU_MAXSIZE	256
+#define	CPU_MAXSIZE	1024
 
 #ifndef	CPU_SETSIZE
 #define	CPU_SETSIZE	CPU_MAXSIZE