gut do_filp_open() a bit more (do_last separation)

Brute-force separation of stuff reachable from do_last: with
the exception of do_link:; just take all that crap to a helper
function as-is and have it tell the caller if it has to go
to do_link.

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