bcachefs: Fix MEAN_AND_VARIANCE kconfig options

Fixes:

https://lore.kernel.org/linux-bcachefs/CAMuHMdXpwMdLuoWsNGa8qacT_5Wv-vSTz0xoBR5n_fnD9cNOuQ@mail.gmail.com/Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/Kconfig

@@ -24,7 +24,6 @@ config BCACHEFS_FS
 	select XXHASH
 	select SRCU
 	select SYMBOLIC_ERRNAME
-	select MEAN_AND_VARIANCE
 	help
 	The bcachefs filesystem - a modern, copy on write filesystem, with
 	support for multiple devices, compression, checksumming, etc.
@@ -78,7 +77,7 @@ config BCACHEFS_NO_LATENCY_ACCT
 config MEAN_AND_VARIANCE_UNIT_TEST
 	tristate "mean_and_variance unit tests" if !KUNIT_ALL_TESTS
 	depends on KUNIT
-	select MEAN_AND_VARIANCE
+	depends on BCACHEFS_FS
 	default KUNIT_ALL_TESTS
 	help
 	  This option enables the kunit tests for mean_and_variance module.