[BUG] x86/sgx: missing kref_put() in sgx_encl_mm_add() error path

[Dingisoul]

Hi Kernel maintainers,

We found a possible refcount leak in sgx_encl_mm_add().

In this function, a reference to encl->refcount is taken before registering the 
MMU notifier:

/* Grab a refcount for the encl_mm->encl reference: */
kref_get(&encl->refcount);  // 1. Reference acquired here.
encl_mm->encl = encl;

ret = __mmu_notifier_register(&encl_mm->mmu_notifier, mm);
if (ret) {
        kfree(encl_mm);     
        return ret;         // 2. Returns without kref_put.
}

If __mmu_notifier_register() fails, the function frees encl_mm but does not 
drop the reference acquired by kref_get(&encl->refcount). This seems to leak 
one reference to encl.

Please let us know if the kref_put is unnecessary here.

Thanks.