iommu: Avoid copying the user array twice in the full-array copy helper

iommu_copy_struct_from_full_user_array() copies a whole user array into a
kernel buffer. In the common case, where user entry_len equals destination
entry size, it takes a fast path and copies the whole array with a single
copy_from_user().

That fast path does not return, so it falls through into the item-by-item
copy_struct_from_user() loop and copies every entry a second time. For an
equal entry_len that loop is just a copy_from_user() of the same bytes, so
the whole array is copied twice for no benefit.

Return right after the bulk copy. The per-item loop then runs only on the
slow path, where entry_len differs and each entry needs size adaption.

Fixes: 4f2e59ccb6 ("iommu: Add iommu_copy_struct_from_full_user_array helper")
Link: https://patch.msgid.link/r/6c9eca4ff584cb977661e97799ac6fe934e7f51c.1780521606.git.nicolinc@nvidia.com
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

include/linux/iommu.h

@@ -547,6 +547,7 @@ iommu_copy_struct_from_full_user_array(void *kdst, size_t kdst_entry_size,
 				   user_array->entry_num *
 					   user_array->entry_len))
 			return -EFAULT;
+		return 0;
 	}
 
 	/* Copy item by item */