tmpfs: add "tmpfs" to the Kconfig prompt to make it obvious.

Add the leading word "tmpfs" to the Kconfig string to make it blindingly
obvious that this selection refers to tmpfs.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>