Instead of allocating the erofs_sb_info in fill_super() allocate it during
erofs_init_fs_context() and ensure that erofs can always have the info
available during erofs_kill_sb(). After this erofs_fs_context is no longer
needed, replace ctx with sbi, no functional changes.
Suggested-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>