bcachefs: Put erasure coding behind an EXPERIMENTAL kconfig option

We still have disk space accounting changes coming for erasure coding,
and the changes won't be as strictly backwards compatible as they'd
ought to be - specifically, we need to start accounting striped data
under a separate counter in bch_alloc (which describes buckets).

A fsck will suffice for upgrading/downgrading, but since erasure coding
is the most incomplete major feature of bcachefs it still makes sense to
put behind a separate kconfig option, so that users are fully aware.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/Kconfig

@@ -33,6 +33,18 @@ config BCACHEFS_QUOTA
 	depends on BCACHEFS_FS
 	select QUOTACTL
 
+config BCACHEFS_ERASURE_CODING
+	bool "bcachefs erasure coding (RAID5/6) support (EXPERIMENTAL)"
+	depends on BCACHEFS_FS
+	select QUOTACTL
+	help
+	This enables the "erasure_code" filesysystem and inode option, which
+	organizes data into reed-solomon stripes instead of ordinary
+	replication.
+
+	WARNING: this feature is still undergoing on disk format changes, and
+	should only be enabled for testing purposes.
+
 config BCACHEFS_POSIX_ACL
 	bool "bcachefs POSIX ACL support"
 	depends on BCACHEFS_FS

fs/bcachefs/alloc_foreground.c

@@ -1321,6 +1321,9 @@ int bch2_alloc_sectors_start_trans(struct btree_trans *trans,
 	int ret;
 	int i;
 
+	if (!IS_ENABLED(CONFIG_BCACHEFS_ERASURE_CODING))
+		erasure_code = false;
+
 	BUG_ON(flags & BCH_WRITE_ONLY_SPECIFIED_DEVS);
 
 	BUG_ON(!nr_replicas || !nr_replicas_required);