staging/lustre: Declare lprocfs_write_frac_u64_helper argument as __user

The buffer that lprocfs_write_frac_u64_helper is working on is
presumed to be a userspace one, so it's not suitable for use
on kernel buffers.

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