alpha: don't bother with __access_ok() in traps.c

	we want to check that address is below TASK_SIZE; sure,
__access_ok(addr, 0, USER_DS) will do that, but it's more straightforward
to just spell it out and that way we can get rid of the damn 'segment'
argument of __access_ok().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>