fanotify: Allow file handle encoding for unhashed events

Allow passing a NULL hash to fanotify_encode_fh and avoid calculating
the hash if not needed.

Link: https://lore.kernel.org/r/20211025192746.66445-15-krisman@collabora.com


Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>