dump_user_range() is used to copy the user page to a coredump file, but if
a hardware memory error occurred during copy, which called from
__kernel_write_iter() in dump_user_range(), it crashes,

  CPU: 112 PID: 7014 Comm: mca-recover Not tainted 6.3.0-rc2 #425

  pc : __memcpy+0x110/0x260
  lr : _copy_from_iter+0x3bc/0x4c8
  ...
  Call trace:
   __memcpy+0x110/0x260
   copy_page_from_iter+0xcc/0x130
   pipe_write+0x164/0x6d8
   __kernel_write_iter+0x9c/0x210
   dump_user_range+0xc8/0x1d8
   elf_core_dump+0x308/0x368
   do_coredump+0x2e8/0xa40
   get_signal+0x59c/0x788
   do_signal+0x118/0x1f8
   do_notify_resume+0xf0/0x280
   el0_da+0x130/0x138
   el0t_64_sync_handler+0x68/0xc0
   el0t_64_sync+0x188/0x190

Generally, the '->write_iter' of file ops will use copy_page_from_iter()
and copy_page_from_iter_atomic(), change memcpy() to copy_mc_to_kernel()
in both of them to handle #MC during source read, which stop coredump
processing and kill the task instead of kernel panic, but the source
address may not always a user address, so introduce a new copy_mc flag in
struct iov_iter{} to indicate that the iter could do a safe memory copy,
also introduce the helpers to set/cleck the flag, for now, it's only used
in coredump's dump_user_range(), but it could expand to any other
scenarios to fix the similar issue.

Link: https://lkml.kernel.org/r/20230417045323.11054-1-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Tong Tiangen <tongtiangen@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>