Staging: use new REQ_FLUSH flag, fix build breakage

REQ_TYPE_LINUX_BLOCK and REQ_LB_OP_FLUSH doesn't exist anymore. Using
the new REQ_FLUSH flag instead

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>