gfs2_atomic_open(): simplify the use of finish_no_open()

In ->atomic_open(inode, dentry, file, opened) calling finish_no_open(file, NULL)
is equivalent to dget(dentry); return finish_no_open(file, dentry);

No need to open-code that...

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