staging/lustre/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().

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3487
Lustre-change: http://review.whamcloud.com/6735


Signed-off-by: John L. Hammond <john.hammond@intel.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>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>