exfat: drain dirty pages after forced shutdown

A forced shutdown sets EXFAT_FLAGS_SHUTDOWN, after which
exfat_writepages() returns -EIO.  Pages dirtied before the shutdown can
then never be written back, so a writer throttled in
balance_dirty_pages() while extending valid_size sleeps forever.

Stuck stack:
  balance_dirty_pages+0x4f0/0xb40
  exfat_prepare_valid_range+0x43/0x140
  exfat_file_write_iter+0x413/0x520
  iter_file_splice_write+0x32d/0x5a0
  direct_splice_actor+0x51/0x270
  splice_direct_to_actor+0xf5/0x270
  do_splice_direct+0x76/0xc0
  vfs_copy_file_range+0x244/0x710
  __do_sys_copy_file_range+0xe6/0x240
  do_syscall_64+0x106/0x5f0
  entry_SYSCALL_64_after_hwframe+0x77/0x7f

Do not reject writeback up front.  Reject it per folio from
->writeback_range() instead: iomap then ends writeback and clears the
dirty state without issuing I/O, which lets the dirty limits drain and
unblocks the writer.

Write syscalls and page faults are still rejected in
exfat_file_write_iter() and exfat_page_mkwrite().  A write that is
already in progress may continue to dirty folios; writeback retires
them without I/O as described above.

Fixes: f761fcdd289d0 ("exfat: Implement sops->shutdown and ioctl")
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>