IB/hfi1: Use kzalloc() for mmu_rb_handler allocation

The code currently assumes that the mmu_notifier struct
embedded in mmu_rb_handler only contains two fields.

There are now extra fields:

struct mmu_notifier {
        struct hlist_node hlist;
        const struct mmu_notifier_ops *ops;
        struct mm_struct *mm;
        struct rcu_head rcu;
        unsigned int users;
};

Given that there in no init for the mmu_notifier, a kzalloc() should
be used to insure that any newly added fields are given a predictable
initial value of zero.

Fixes: 06e0ffa6 ("IB/hfi1: Re-factor MMU notification code")
Link: https://lore.kernel.org/r/1617026056-50483-9-git-send-email-dennis.dalessandro@cornelisnetworks.com


Reviewed-by: Adam Goldman <adam.goldman@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>