NFSv4: Remove unreachable error condition due to rpc_run_task()

nfs4_proc_layoutget() invokes rpc_run_task(), which return the value to
"task". Since rpc_run_task() is impossible to return an ERR pointer,
there is no need to add the IS_ERR() condition on "task" here. So we
need to remove it.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>