fs: s/__u32/u32/ for s_fsnotify_mask

The underscore variants are for uapi whereas the non-underscore variants
are for in-kernel consumers.

Link: https://lore.kernel.org/r/20240822-anwerben-nutzung-1cd6c82a565f@brauner


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>