riscv/mm/fault: Move vmalloc fault handling to vmalloc_fault()

This patch moves the vmalloc fault handling in do_page_fault() to
vmalloc_fault() function and converts gotos to calls to the new
function.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>