rds: Fix min() warning in rds_message_inc_copy_to_user()

net/rds/message.c: In function ‘rds_message_inc_copy_to_user’:
net/rds/message.c:328: warning: comparison of distinct pointer types lacks a cast

Use min_t(unsigned long, ...) like is done in
rds_message_copy_from_user().

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>