bcachefs: Fix determining required file handle length

The ->encode_fh method is responsible for setting amount of space
required for storing the file handle if not enough space was provided.
bch2_encode_fh() was not setting required length in that case which
breaks e.g. fanotify. Fix it.

Reported-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>