ceph: mark directory as non-complete after loading key

When setting a directory's crypt context, ceph_dir_clear_complete()
needs to be called otherwise if it was complete before, any existing
(old) dentry will still be valid.

This patch adds a wrapper around __fscrypt_prepare_readdir() which will
ensure a directory is marked as non-complete if key status changes.

[ xiubli: revise commit title per Milind ]

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>