ceph: don't clear I_NEW until inode metadata is fully populated

Currently, we could have an open-by-handle (or NFS server) call
into the filesystem and start working with an inode before it's
properly filled out.

Don't clear I_NEW until we have filled out the inode, and discard it
properly if that fails. Note that we occasionally take an extra
reference to the inode to ensure that we don't put the last reference in
discard_new_inode, but rather leave it for ceph_async_iput.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>