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.com


Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>