fsinfo: Export superblock notification counter

Provide an fsinfo attribute to export the superblock notification counter
so that it can be polled in the case of a notification buffer overrun.
This is accessed with:

	struct fsinfo_params params = {
		.request = FSINFO_ATTR_SB_NOTIFICATIONS,
	};

and returns a structure that looks like:

	struct fsinfo_sb_notifications {
		__u64	watch_id;
		__u32	notify_counter;
		__u32	__reserved[1];
	};

Where watch_id is a number uniquely identifying the superblock in
notification records and notify_counter is incremented for each
superblock notification posted.

Signed-off-by: David Howells <dhowells@redhat.com>
6 files changed