bcachefs: Fix pool->alloc NULL pointer dereference

bcachefs: Fix pool->alloc NULL pointer dereference

btree_interior_update_pool has not been initialized before the
filesystem becomes read-write, thus mempool_alloc in bch2_btree_update_start
will trigger pool->alloc NULL pointer dereference in mempool_alloc_noprof

Reported-by: syzbot+2f3859bd28f20fa682e6@syzkaller.appspotmail.com
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>