x86/sgx: Use vmalloc_array() instead of vmalloc()

Use vmalloc_array() instead of vmalloc() to calculate the number of
bytes to allocate.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/all/20241112182633.172944-2-thorsten.blum%40linux.dev