erofs: open via dedicated fs bdev helpers

Route opens through fs_bdev_file_open_by_path() so each external device
is registered against the correct superblock, and convert the matching
releases.

Gao Xiang: I think typical immutable filesystems don't need .shutdown()
and .remove_bdev() for the following reasons:

  - blk_mark_disk_dead() sets GD_DEAD in advance of fs_bdev_mark_dead()
    so that the following bios will fail immediately; block_device
    references are still valid so it seems overkill to handle dead
    blockdevs in the deep filesystem I/O submission path.

  - Immutable filesystems like EROFS don't have write paths and journals,
    so they don't need to block writes (i.e., new dirty pages), metadata
    changes, and abort journals.

  - The comment above loop_change_fd() documents a valid read-only use
    case we need to support anyway, but it calls disk_force_media_change()
    which will call fs_bdev_mark_dead() later: we don't want loop_change_fd()
    shutdowns the active filesystems and return -EIO unconditionally.

Currently I think the default behavior (shrink_dcache_sb + evict_inodes)
in fs_bdev_mark_dead() is enough for immutable filesystems, tried to
document in the commit here for later reference.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-14-7df6b864028e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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