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 <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>