mm, swap: restore swap_space attr aviod kernel panic

commit 8b47299a411a ("mm, swap: mark swap address space ro and add context
debug check") made the swap address space read-only. It may lead to
kernel panic if arch_prepare_to_swap returns a failure under heavy memory
pressure as follows,

el1_abort+0x40/0x64
el1h_64_sync_handler+0x48/0xcc
el1h_64_sync+0x84/0x88
errseq_set+0x4c/0xb8 (P)
__filemap_set_wb_err+0x20/0xd0
shrink_folio_list+0xc20/0x11cc
evict_folios+0x1520/0x1be4
try_to_shrink_lruvec+0x27c/0x3dc
shrink_one+0x9c/0x228
shrink_node+0xb3c/0xeac
do_try_to_free_pages+0x170/0x4f0
try_to_free_pages+0x334/0x534
__alloc_pages_direct_reclaim+0x90/0x158
__alloc_pages_slowpath+0x334/0x588
__alloc_frozen_pages_noprof+0x224/0x2fc
__folio_alloc_noprof+0x14/0x64
vma_alloc_zeroed_movable_folio+0x34/0x44
do_pte_missing+0xad4/0x1040
handle_mm_fault+0x4a4/0x790
do_page_fault+0x288/0x5f8
do_translation_fault+0x38/0x54
do_mem_abort+0x54/0xa8

Restore swap address space as not ro to avoid the panic.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 8b47299a411a ("mm, swap: mark swap address space ro and add context debug check")
Signed-off-by: robin.kuo <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: andrew.yang <[e-mail]>
Cc: AngeloGiaocchino Del Regno <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Barry Song <[e-mail]>
Cc: Chinwen Chang <[e-mail]>
Cc: Chris Li <[e-mail]>
Cc: Kairui Song <[e-mail]>
Cc: Kairui Song <[e-mail]>
Cc: Kemeng Shi <[e-mail]>
Cc: Mathias Brugger <[e-mail]>
Cc: Nhat Pham <[e-mail]>
Cc: Qun-wei Lin <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/swap.h

···
 void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
 /* linux/mm/swap_state.c */
-extern struct address_space swap_space __ro_after_init;
+extern struct address_space swap_space __read_mostly;
 static inline struct address_space *swap_address_space(swp_entry_t entry)
 {
 	return &swap_space;

mm/swap_state.c

···
 #endif
 };
 
-/* Set swap_space as read only as swap cache is handled by swap table */
-struct address_space swap_space __ro_after_init = {
+struct address_space swap_space __read_mostly = {
 	.a_ops = &swap_aops,
 };