LU-3487 llite: handle io init failure in ll_fault_io_init()

In ll_fault_io_init(), if cl_io_init() has failed then cleanup and
return an ERR_PTR(). This fixes an oops in the page fault handling
code when a partially initialized io is used. In ll_page_mkwrite0() do
not call cl_io_fini() on an ERR_PTR().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I04b1c1f3071413ba3fa53bbbfcb83fe325ed9050
Reviewed-on: http://review.whamcloud.com/6735
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>