Add sample notification program

The sample program is run like:

	./samples/watch_queue/watch_test

and watches "/" for mount changes and the current session keyring for key
changes:

	# keyctl add user a a @s
	1035096409
	# keyctl unlink 1035096409 @s

producing:

	# ./watch_test
	read() = 16
	NOTIFY[000]: ty=000001 sy=02 i=00000110
	KEY 2ffc2e5d change=2[linked] aux=1035096409
	read() = 16
	NOTIFY[000]: ty=000001 sy=02 i=00000110
	KEY 2ffc2e5d change=3[unlinked] aux=1035096409

Other events may be produced, such as with a failing disk:

	read() = 22
	NOTIFY[000]: ty=000003 sy=02 i=00000416
	USB 3-7.7 dev-reset e=0 r=0
	read() = 24
	NOTIFY[000]: ty=000002 sy=06 i=00000418
	BLOCK 00800050 e=6[critical medium] s=64000ef8

This corresponds to:

	blk_update_request: critical medium error, dev sdf, sector 1677725432 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0

in dmesg.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/samples/Makefile b/samples/Makefile
index f8f847b4..8617edf 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -24,3 +24,4 @@
 obj-y					+= vfio-mdev/
 subdir-$(CONFIG_SAMPLE_VFS)		+= vfs
 obj-$(CONFIG_SAMPLE_INTEL_MEI)		+= mei/
+subdir-$(CONFIG_SAMPLE_WATCH_QUEUE)	+= watch_queue