io_uring/memmap: don't use page_address() on a highmem page

For older/32-bit systems with highmem, don't assume that the pages in
a mapped region are always going to be mapped. If io_region_init_ptr()
finds that the pages are coalescable, also check if the first page is
a HighMem page or not. If it is, fall through to the usual vmap()
mapping rather than attempt to get the unmapped page address.

Cc: stable@vger.kernel.org
Fixes: c4d0ac1c ("io_uring/memmap: optimise single folio regions")
Link: https://lore.kernel.org/all/681fe2fb.050a0220.f2294.001a.GAE@google.com/


Reported-by:  <syzbot+5b8c4abafcb1d791ccfc@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/681fed0a.050a0220.f2294.001c.GAE@google.com/


Reported-by:  <syzbot+6456a99dfdc2e78c4feb@syzkaller.appspotmail.com>
Tested-by:  <syzbot+6456a99dfdc2e78c4feb@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>