atm: separate ATM_GETNAMES handling from the rest of atm_dev_ioctl()

atm_dev_ioctl() does copyin in two different ways - one for
ATM_GETNAMES, another for everything else.  Start with separating
the former into a new helper (atm_getnames()).  The next step
will be to lift the copyin into the callers.

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