Use a platform-independent constant for PKG_MAX_SIZE.

This constant determines the number of rights libnv will attempt to
transmit in a given control message.  In practice, the upper limit
defined by the kernel is machine-dependent and is smaller on 64-bit
kernels than on 32-bit kernels.  To ensure that a 32-bit libnv works
as expected when run on a 64-bit kernel, use a limit that will work
on both 32-bit and 64-bit kernels.

PR:		238511
Discussed with:	oshogbo
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20942

lib/libnv/msgio.c

@@ -63,7 +63,13 @@ __FBSDID("$FreeBSD$");
 /* Linux: arbitrary size, but must be lower than SCM_MAX_FD. */
 #define	PKG_MAX_SIZE	((64U - 1) * CMSG_SPACE(sizeof(int)))
 #else
-#define	PKG_MAX_SIZE	(MCLBYTES / CMSG_SPACE(sizeof(int)) - 1)
+/*
+ * To work around limitations in 32-bit emulation on 64-bit kernels, use a
+ * machine-independent limit on the number of FDs per message.  Each control
+ * message contains 1 FD and requires 12 bytes for the header, 4 pad bytes,
+ * 4 bytes for the descriptor, and another 4 pad bytes.
+ */
+#define	PKG_MAX_SIZE	(MCLBYTES / 24)
 #endif
 
 static int