bcachefs: Mark bch_inode_info as SLAB_ACCOUNT

After commit 230e9fc2 ("slab: add SLAB_ACCOUNT flag"), we need to mark
the inode cache as SLAB_ACCOUNT, similar to commit 5d097056

 ("kmemcg:
account for certain kmem allocations to memcg")

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>