bcachefs: allocate inode by using alloc_inode_sb()

The inode allocation is supposed to use alloc_inode_sb(), so convert
kmem_cache_alloc() to alloc_inode_sb().

It will also fix [1] to avoid the NULL pointer dereference BUG in
list_lru_add() when CONFIG_MEMCG is enabled.

Links:
[1]: https://lore.kernel.org/all/20589721-46c0-4344-b2ef-6ab48bbe2ea5@linux.dev/
[2]: https://lore.kernel.org/all/7db60e36-9c96-4938-a28d-a9745e287386@linux.dev/



Fixes: 86d81ec5 ("bcachefs: Mark bch_inode_info as SLAB_ACCOUNT")
Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>