bcachefs: add missing selection of XARRAY_MULTI

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: be212d8 ("bcachefs: bs > ps support")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: ebiggers

fs/bcachefs/Kconfig

@@ -26,6 +26,7 @@ config BCACHEFS_FS
 	select SRCU
 	select SYMBOLIC_ERRNAME
 	select MIN_HEAP
+	select XARRAY_MULTI
 	help
 	The bcachefs filesystem - a modern, copy on write filesystem, with
 	support for multiple devices, compression, checksumming, etc.