fs: create {sb,file}_write_not_started() helpers

Create new helpers {sb,file}_write_not_started() that can be used
to assert that sb_start_write() is not held.

This is needed for fanotify "pre content" events.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-17-amir73il@gmail.comReviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/fs.h

@@ -1670,6 +1670,17 @@ static inline bool sb_write_started(const struct super_block *sb)
 	return __sb_write_started(sb, SB_FREEZE_WRITE);
 }
 
+/**
+ * sb_write_not_started - check if SB_FREEZE_WRITE is not held
+ * @sb: the super we write to
+ *
+ * May be false positive with !CONFIG_LOCKDEP/LOCK_STATE_UNKNOWN.
+ */
+static inline bool sb_write_not_started(const struct super_block *sb)
+{
+	return __sb_write_started(sb, SB_FREEZE_WRITE) <= 0;
+}
+
 /**
  * file_write_started - check if SB_FREEZE_WRITE is held
  * @file: the file we write to
@@ -1685,6 +1696,21 @@ static inline bool file_write_started(const struct file *file)
 	return sb_write_started(file_inode(file)->i_sb);
 }
 
+/**
+ * file_write_not_started - check if SB_FREEZE_WRITE is not held
+ * @file: the file we write to
+ *
+ * May be false positive with !CONFIG_LOCKDEP/LOCK_STATE_UNKNOWN.
+ * May be false positive with !S_ISREG, because file_start_write() has
+ * no effect on !S_ISREG.
+ */
+static inline bool file_write_not_started(const struct file *file)
+{
+	if (!S_ISREG(file_inode(file)->i_mode))
+		return true;
+	return sb_write_not_started(file_inode(file)->i_sb);
+}
+
 /**
  * sb_end_write - drop write access to a superblock
  * @sb: the super we wrote to