scripts/gdb/vmalloc: disable on no-MMU

vmap_area does not exist on no-MMU, therefore the GDB scripts fail to
load:

Traceback (most recent call last):
  File "<...>/vmlinux-gdb.py", line 51, in <module>
    import linux.vmalloc
  File "<...>/scripts/gdb/linux/vmalloc.py", line 14, in <module>
    vmap_area_ptr_type = vmap_area_type.get_type().pointer()
                         ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<...>/scripts/gdb/linux/utils.py", line 28, in get_type
    self._type = gdb.lookup_type(self._name)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
gdb.error: No struct type named vmap_area.

To fix this, disable the command and add an informative error message if
CONFIG_MMU is not defined, following the example of lx-slabinfo.

Link: https://lkml.kernel.org/r/20231031202235.2655333-2-ben.wolsieffer@hefring.com
Fixes: 852622b ("scripts/gdb/vmalloc: add vmallocinfo support")
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: lopsided98

scripts/gdb/linux/constants.py.in

@@ -158,3 +158,4 @@ LX_CONFIG(CONFIG_STACKDEPOT)
 LX_CONFIG(CONFIG_PAGE_OWNER)
 LX_CONFIG(CONFIG_SLUB_DEBUG)
 LX_CONFIG(CONFIG_SLAB_FREELIST_HARDENED)
+LX_CONFIG(CONFIG_MMU)

scripts/gdb/linux/vmalloc.py

@@ -10,8 +10,9 @@
 import re
 from linux import lists, utils, stackdepot, constants, mm
 
-vmap_area_type = utils.CachedType('struct vmap_area')
-vmap_area_ptr_type = vmap_area_type.get_type().pointer()
+if constants.LX_CONFIG_MMU:
+    vmap_area_type = utils.CachedType('struct vmap_area')
+    vmap_area_ptr_type = vmap_area_type.get_type().pointer()
 
 def is_vmalloc_addr(x):
     pg_ops = mm.page_ops().ops
@@ -25,6 +26,9 @@ def __init__(self):
         super(LxVmallocInfo, self).__init__("lx-vmallocinfo", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
+        if not constants.LX_CONFIG_MMU:
+            raise gdb.GdbError("Requires MMU support")
+
         vmap_area_list = gdb.parse_and_eval('vmap_area_list')
         for vmap_area in lists.list_for_each_entry(vmap_area_list, vmap_area_ptr_type, "list"):
             if not vmap_area['vm']: