btrfs: constify static arrays

There are a few statically initialized arrays that can be made const.
The remaining (like file_system_type, sysfs attributes or prop handlers)
do not allow that due to type mismatch when passed to the APIs or
because the structures are modified through other members.

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index da94138..fd429d7 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -5286,7 +5286,7 @@
 static int btrfs_ioctl_get_supported_features(struct file *file,
 					      void __user *arg)
 {
-	static struct btrfs_ioctl_feature_flags features[3] = {
+	static const struct btrfs_ioctl_feature_flags features[3] = {
 		INIT_FEATURE_FLAGS(SUPP),
 		INIT_FEATURE_FLAGS(SAFE_SET),
 		INIT_FEATURE_FLAGS(SAFE_CLEAR)