->atomic_open(): return 0 in all success cases

FMODE_OPENED can be used to distingusish "successful open" from the
"called finish_no_open(), do it yourself" cases.  Since finish_no_open()
has been adjusted, no changes in the instances were actually needed.
The caller has been adjusted.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>