Provide kern.maxphys sysctl, which returns MAXPHYS. Naming matches NetBSD.

sys/kern/kern_mib.c

@@ -138,6 +138,9 @@ char kernelname[MAXPATHLEN] = "/kernel";	/* XXX bloat */
 SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
     kernelname, sizeof kernelname, "Name of kernel file booted");
 
+SYSCTL_INT(_kern, KERN_MAXPHYS, maxphys, CTLFLAG_RD | CTLFLAG_CAPRD,
+    SYSCTL_NULL_INT_PTR, MAXPHYS, "Maximum block I/O access size");
+
 SYSCTL_INT(_hw, HW_NCPU, ncpu, CTLFLAG_RD|CTLFLAG_CAPRD,
     &mp_ncpus, 0, "Number of active CPUs");
 

sys/sys/sysctl.h

@@ -835,6 +835,7 @@ TAILQ_HEAD(sysctl_ctx_list, sysctl_ctx_entry);
 #define	KERN_IOV_MAX		35	/* int: value of UIO_MAXIOV */
 #define	KERN_HOSTUUID		36	/* string: host UUID identifier */
 #define	KERN_ARND		37	/* int: from arc4rand() */
+#define	KERN_MAXPHYS		38	/* int: MAXPHYS value */
 /*
  * KERN_PROC subtypes
  */