xen/arm: alternative: Don't call vmap() within stop_machine_run()

xen/arm: alternative: Don't call vmap() within stop_machine_run()

Commit 88a037e2cfe1 "page_alloc: assert IRQs are enabled in heap
alloc/free" extended the checks in the buddy allocator to catch
any use of the helpers from context with interrupts disabled.

Unfortunately, the rule is not followed in the alternative code and
this will result to crash at boot with debug enabled:

(XEN) Xen call trace:
(XEN)    [<0022a510>] alloc_xenheap_pages+0x120/0x150 (PC)
(XEN)    [<00000000>] 00000000 (LR)
(XEN)    [<002736ac>] arch/arm/mm.c#xen_pt_update+0x144/0x6e4
(XEN)    [<002740d4>] map_pages_to_xen+0x10/0x20
(XEN)    [<00236864>] __vmap+0x400/0x4a4
(XEN)    [<0026aee8>] arch/arm/alternative.c#__apply_alternatives_multi_stop+0x144/0x1ec
(XEN)    [<0022fe40>] stop_machine_run+0x23c/0x300
(XEN)    [<002c40c4>] apply_alternatives_all+0x34/0x5c
(XEN)    [<002ce3e8>] start_xen+0xcb8/0x1024
(XEN)    [<00200068>] arch/arm/arm32/head.o#primary_switched+0xc/0x1c

The interrupts will be disabled by the state machine in stop_machine_run(),
hence why the ASSERT is hit.

For now the patch extending the checks has been reverted, but it would
be good to re-introduce it (allocation with interrupts disabled is not
desirable).

So move the re-mapping of Xen to the caller of stop_machine_run().

Signed-off-by: Julien Grall <jgrall@amazon.com>
Cc: David Vrabel <dvrabel@amazon.co.uk>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>