btrfs-progs: add warning helper for experimental build

We should warn that there's an experimental feature used. Add a helper
with optional description. Should be used only if such feature is used
and not always.

Signed-off-by: David Sterba <dsterba@suse.com>

common/utils.c

@@ -1152,6 +1152,14 @@ int btrfs_warn_multiple_profiles(int fd)
 	return 1;
 }
 
+void btrfs_warn_experimental(const char *str)
+{
+#if EXPERIMENTAL
+	warning("Experimental build with unstable or unfinished features");
+	warning_on(str != NULL, "%s\n", str);
+#endif
+}
+
 /*
  * Open a file in fsid directory in sysfs and return the file descriptor or
  * error

common/utils.h

@@ -110,6 +110,7 @@ void init_rand_seed(u64 seed);
 
 char *btrfs_test_for_multiple_profiles(int fd);
 int btrfs_warn_multiple_profiles(int fd);
+void btrfs_warn_experimental(const char *str);
 
 int sysfs_open_file(const char *name);
 int sysfs_open_fsid_file(int fd, const char *filename);