flex_array: remove unneeded index calculation

flex_array_get() calculates an index value, then drops it on the floor;
simply remove it.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>