simplify gfs2_atomic_open()

the difference from 9p et.al. is that on gfs2 the lookup side might
end up opening the file.  That's what the FMODE_OPENED check there
is about - and it might actually be seen with finish_open() having
failed, if it fails late enough.

Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>