selftests: move dnotify_test from Documentation/filesystems

Move dnotify_test.c, Makefile, and .gitignore from Documentation/filesystems
to selftests/filesystems.

Remove filesystems build target from Documentation/Makefile and update
selftests/filesystems/Makefile to work under selftests. dnotify_test will
not be run as part of selftests suite and will not be included in install
targets. It can be built separately for now.

Acked-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Documentation/Makefile

···
 subdir-y := accounting auxdisplay blackfin \
-	filesystems filesystems ia64 laptops mic misc-devices \
+	ia64 laptops mic misc-devices \
 	networking pcmcia prctl ptp timers vDSO watchdog

Documentation/filesystems/Makefile

···
-# List of programs to build
-hostprogs-y := dnotify_test
-
-# Tell kbuild to always build the programs
-always := $(hostprogs-y)

tools/testing/selftests/filesystems/Makefile

···
+TEST_PROGS := dnotify_test
+all: $(TEST_PROGS)
+
+include ../lib.mk
+
+clean:
+	rm -fr $(TEST_PROGS)