Add an ->atomic_open implementation which replaces the atomic open+create
operation implemented via ->create.  No functionality is changed.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
CC: Eric Van Hensbergen <ericvh@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>