When CONFIG_XARRAY_MULTI is not set, reading from a bcachefs file hits
the 'BUG_ON(order > 0);' in xas_set_order(), because it tries to insert
a large folio in the page cache. Fix this by making bcachefs select
XARRAY_MULTI.
Fixes: be212d86b19c ("bcachefs: bs > ps support") Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>