staging: lustre: fix min() comparison types lacks cast build warning

Recent changes to use the builtin min functions [1] introduced
type checking which wasn't present before.  This resulted in
"comparision of distinct pointer types lacks a cast" build
warnings on non X86 architectures [2,3].

  [1]: https://lkml.org/lkml/2014/12/25/145
  [2]: https://lists.01.org/pipermail/kbuild-all/2015-January/008588.html
  [3]: https://lists.01.org/pipermail/kbuild-all/2015-January/008589.html



The call to min() which resulted in this warning took the result
of kiblnd_rd_frag_size(), which returned a __u32, and the
variable 'resid', which is an int.  'resid' is inside a while
loop which is only entered if it is positive.  Casting it as a
__u32 can be perormed without a loss of data or change in
functionality.

Fix the warning by casting 'resid' as a __u32.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>