hugetlbfs: add Kconfig help text

In kernel bugzilla #6248 (http://bugzilla.kernel.org/show_bug.cgi?id=6248

),
Adrian Bunk <bunk@stusta.de> notes that CONFIG_HUGETLBFS is missing Kconfig
help text.

Signed-off-by: Arthur Othieno <apgo@patchbomb.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>