atomic_open(): saner calling conventions (return dentry on success)

Currently it either returns -E... or puts (nd->path.mnt,dentry)
into *path and returns 0.  Make it return ERR_PTR(-E...) or
dentry; adjust the caller.  Fewer arguments and it's easier
to keep track of *path contents that way.

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