ceph: switch ceph_lookup/atomic_open() to use new fscrypt helper

Instead of setting the no-key dentry, use the new
fscrypt_prepare_lookup_partial() helper.  We still need to mark the
directory as incomplete if the directory was just unlocked.

In ceph_atomic_open() this fixes a bug where a dentry is incorrectly
set with DCACHE_NOKEY_NAME when 'dir' has been evicted but the key is
still available (for example, where there's a drop_caches).

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>