io_uring: Fix unsigned 'res' comparison with zero in io_fixup_rw_res()

Smatch warning: io_fixup_rw_res() warn:
	unsigned 'res' is never less than zero.

Change type of 'res' from unsigned to long.

Fixes: d6b7efc7 ("io_uring/rw: fix error'ed retry return values")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>