thunderbolt: debugfs: Replace get_zeroed_page() with kzalloc()

validate_and_copy_from_user() allocates a page to store data from
userspace via get_zeroed_page(), and then returns it as a buffer.
Neither the function itself nor its callers require struct page access.

This buffer can easily be allocated with kzalloc() as there is nothing
special about it that requires going through the page allocator.
kzalloc(), which internally reduces to kmalloc() with __GFP_ZERO,
provides a better API and kfree() does not need to know the size of the
freed object. Additionally it removes the casts of (void *) and
(unsigned long) which only obfuscate the code.

Replace get_zeroed_page() with kzalloc() and free_page() with kfree().

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com/
Signed-off-by: Mahad Ibrahim <mahad.ibrahim.dev@gmail.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>