erofs: reliably distinguish block based and fscache mode

When erofs_kill_sb() is called in block dev based mode, s_bdev may not
have been initialised yet, and if CONFIG_EROFS_FS_ONDEMAND is enabled,
it will be mistaken for fscache mode, and then attempt to free an anon_dev
that has never been allocated, triggering the following warning:

============================================
ida_free called for id=0 which is not allocated.
WARNING: CPU: 14 PID: 926 at lib/idr.c:525 ida_free+0x134/0x140
Modules linked in:
CPU: 14 PID: 926 Comm: mount Not tainted 6.9.0-rc3-dirty #630
RIP: 0010:ida_free+0x134/0x140
Call Trace:
 <TASK>
 erofs_kill_sb+0x81/0x90
 deactivate_locked_super+0x35/0x80
 get_tree_bdev+0x136/0x1e0
 vfs_get_tree+0x2c/0xf0
 do_new_mount+0x190/0x2f0
 [...]
============================================

Now when erofs_kill_sb() is called, erofs_sb_info must have been
initialised, so use sbi->fsid to distinguish between the two modes.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20240419123611.947084-3-libaokun1@huawei.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>