bcachefs: Do not check folio_has_private()

folio_has_private() is an attractive nuisance; filesystem authors
generally don't realise that it actually checks two flags (one of which
is never set by bcachefs).  There's no need to check the private flag at
all; for folios owned by bcachefs, we know that folio->private is NULL
when the private flag is clear and non-NULL when the private flag is set.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>