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: 852622bf

 ("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>