staging: lustre: ptlrpc: lproc_ptlrpc.c - fix dereferenceing user space buffer

- this fixes sparse warning for directly deferencing user space buffer

drivers/staging/lustre/lustre/ptlrpc/lproc_ptlrpc.c:652:33: warning: incorrect type in argument 2 (different address spaces)
drivers/staging/lustre/lustre/ptlrpc/lproc_ptlrpc.c:652:33:    expected void const [noderef] <asn:1>*from
drivers/staging/lustre/lustre/ptlrpc/lproc_ptlrpc.c:652:33:    got char const *buffer

Signed-off-by: Anil Belur <askb23@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>