arm64: fix early devmap assertion

The purpose of this KASSERT is to ensure that we do not run out of space
in the early devmap. However, the devmap grew beyond its initial size of
2MB in r336519, and this assertion did not grow with it.

A devmap mapping of a 1080p framebuffer requires 1920x1080 bytes, or
1.977 MB, so it is just barely able to fit without triggering the
assertion, provided no other devices are mapped before it. With the
addition of `options GDB` in GENERIC by bbfa199cbc, the uart is now
mapped for the purposes of a debug port, before mapping the framebuffer.
The presence of both these conditions pushes the selected virtual
address just below the threshold, triggering the assertion.

To fix this, use the correct size of the devmap, defined by
PMAP_MAPDEV_EARLY_SIZE. Since this code is shared with RISC-V, define
it for that platform as well (although it is a different size).

PR:		25241
Reported by:	gbe
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

sys/kern/subr_devmap.c

@@ -275,7 +275,7 @@ pmap_mapdev(vm_offset_t pa, vm_size_t size)
 	if (early_boot) {
 		akva_devmap_vaddr = trunc_page(akva_devmap_vaddr - size);
 		va = akva_devmap_vaddr;
-		KASSERT(va >= VM_MAX_KERNEL_ADDRESS - L2_SIZE,
+		KASSERT(va >= VM_MAX_KERNEL_ADDRESS - PMAP_MAPDEV_EARLY_SIZE,
 		    ("Too many early devmap mappings"));
 	} else
 #endif

sys/riscv/include/vmparam.h

@@ -235,6 +235,7 @@ extern vm_offset_t init_pt_va;
 #define	ZERO_REGION_SIZE	(64 * 1024)	/* 64KB */
 
 #define	DEVMAP_MAX_VADDR	VM_MAX_KERNEL_ADDRESS
+#define	PMAP_MAPDEV_EARLY_SIZE	(L2_SIZE * 2)
 
 /*
  * No non-transparent large page support in the pmap.