staging: lustre: Use __user for a pointer to a user space address

This commit corrects two sparse warnings caused by the lack of a __user
annotation for the third argument of the libcfs_ioctl_handle function.

  module.c:165:68: warning: incorrect type in argument 1
    (different address spaces)
  module.c:165:68:    expected void [noderef] <asn:1>*arg
  module.c:165:68:    got void *arg
  module.c:209:47: warning: incorrect type in argument 3
    (different address spaces)
  module.c:209:47:    expected void *arg
  module.c:209:47:    got void [noderef] <asn:1>*arg

The need to have the __user annotation is supported by the fact that
libcfs_ioctl_handle passes its third argument to a helper function
(libcfs_ioctl_popdata) which also has a __user annotation for its
corresponding argument.

Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>