staging: lustre: declare fiemap_for_stripe static

Declare fiemap_for_stripe as static to fix sparse warnings:

> warning: symbol 'fiemap_for_stripe' was not declared. Should it be
> static?

Signed-off-by: David Wittman <dwittman@gmail.com>
Acked-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>