Staging: lustre: osc: Declare as static

Declare osc_extent_find and osc_unreserve_grant as static since they
are used only in this particular file.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>