fs: create file_write_started() helper

fs: create file_write_started() helper

Convenience wrapper for sb_write_started(file_inode(inode)->i_sb)), which
has a single occurrence in the code right now.

Document the false negatives of those helpers, which makes them unusable
to assert that sb_start_write() is not held.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-16-amir73il@gmail.com
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>