Add 24 more page table pages we allocate on boot-up. 16MB slop

is a little tight in and by itself, but severily insufficient
when one needs to map a large frame buffer as part of console
initialization. 64MB slop should be enough for a while. As an
example: a 15" MacBook Pro with retina display needs ~28MB of
KVA for the frame buffer.

PR:		193745

sys/amd64/amd64/pmap.c

@@ -699,8 +699,14 @@ nkpt_init(vm_paddr_t addr)
 	 * pmap_growkernel() will need to allocate page table pages to map
 	 * the entire 512GB of KVA space which is an unnecessary tax on
 	 * physical memory.
+	 *
+	 * Secondly, device memory mapped as part of setting up the low-
+	 * level console(s) is taken from KVA, starting at virtual_avail.
+	 * This is because cninit() is called after pmap_bootstrap() but
+	 * before vm_init() and pmap_init(). 20MB for a frame buffer is
+	 * not uncommon.
 	 */
-	pt_pages += 8;		/* 16MB additional slop for kernel modules */
+	pt_pages += 32;		/* 64MB additional slop. */
 #endif
 	nkpt = pt_pages;
 }