staging/lustre/libcfs: Copy correct amount in libcfs_ioctl_getdata

Commit b8ff756bc351 ("staging: lustre: libcfs: merge code from
libcfs_ioctl into libcfs_ioctl_getdata") introduced a problem
copying just a single pointer worth of data from userspace
instead of whole libcfs_ioctl_hdr structure.
Adjust the copying amount.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Acked-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>