ceph: abort osd requests on force umount

This avoid force umount waiting on page writeback:

  io_schedule+0xd/0x30
  wait_on_page_bit_common+0xc6/0x130
  __filemap_fdatawait_range+0xbd/0x100
  filemap_fdatawait_keep_errors+0x15/0x40
  sync_inodes_sb+0x1cf/0x240
  sync_filesystem+0x52/0x90
  generic_shutdown_super+0x1d/0x110
  ceph_kill_sb+0x28/0x80 [ceph]
  deactivate_locked_super+0x35/0x60
  cleanup_mnt+0x36/0x70
  task_work_run+0x79/0xa0
  exit_to_usermode_loop+0x62/0x70
  do_syscall_64+0xdb/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
  0xffffffffffffffff
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/super.c

@@ -793,6 +793,7 @@ static void ceph_umount_begin(struct super_block *sb)
 	if (!fsc)
 		return;
 	fsc->mount_state = CEPH_MOUNT_SHUTDOWN;
+	ceph_osdc_abort_requests(&fsc->client->osdc, -EIO);
 	ceph_mdsc_force_umount(fsc->mdsc);
 	return;
 }