fanotify: Support null inode event in fanotify_dfid_inode

fanotify: Support null inode event in fanotify_dfid_inode

FAN_FS_ERROR doesn't support DFID, but this function is still called for
every event.  The problem is that it is not capable of handling null
inodes, which now can happen in case of superblock error events.  For
this case, just returning dir will be enough.

Link: https://lore.kernel.org/r/20211025192746.66445-14-krisman@collabora.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>