staging: lustre: replace MIN with min, cast (__kernel_size_t)

Switch from MIN to min and fix the new type warning.  The
warning is produced because a comparison between iov_len,
which is a __kernel_size_t, is made to kiov_len, which is an
unsigned int (include/linux/lnet/types.h).  Fix the warning
by casting kiov_len to __kernel_size_t.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>