take post-lookup part of do_last() out of loop

now we can have open_last_lookups() directly from the loop in
path_openat() - the rest of do_last() never returns a symlink
to follow, so we can bloody well leave the loop first.

Rename the rest of that thing from do_last() to do_open() and
make it return an int.

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