General notification queue with user mmap()'able ring buffer

Implement a misc device that implements a general notification queue as a
ring buffer that can be mmap()'d from userspace.

The way this is done is:

 (1) An application opens the device and indicates the size of the ring
     buffer that it wants to reserve in pages (this can only be set once):

	fd = open("/dev/watch_queue", O_RDWR);
	ioctl(fd, IOC_WATCH_QUEUE_NR_PAGES, nr_of_pages);

 (2) The application should then map the pages that the device has
     reserved.  Each instance of the device created by open() allocates
     separate pages so that maps of different fds don't interfere with one
     another.  Multiple mmap() calls on the same fd, however, will all work
     together.

	page_size = sysconf(_SC_PAGESIZE);
	mapping_size = nr_of_pages * page_size;
	char *buf = mmap(NULL, mapping_size, PROT_READ|PROT_WRITE,
			 MAP_SHARED, fd, 0);

The ring is divided into 8-byte slots.  Entries written into the ring are
variable size and can use between 1 and 63 slots.  A special entry is
maintained in the first two slots of the ring that contains the head and
tail pointers.  This is skipped when the ring wraps round.  Note that
multislot entries, therefore, aren't allowed to be broken over the end of
the ring, but instead "skip" entries are inserted to pad out the buffer.

Each entry has a 1-slot header that describes it:

	struct watch_notification {
		__u32	type:24;
		__u32	subtype:8;
		__u32	info;
	};

The type indicates the source (eg. mount tree changes, superblock events,
keyring changes, block layer events) and the subtype indicates the event
type (eg. mount, unmount; EIO, EDQUOT; link, unlink).  The info field
indicates a number of things, including the entry length, an ID assigned to
a watchpoint contributing to this buffer, type-specific flags and meta
flags, such as an overrun indicator.

Supplementary data, such as the key ID that generated an event, are
attached in additional slots.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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