mtd: create an mtd_oobavail() helper and make use of it

Currently, all MTD drivers/sublayers exposing an OOB area are
doing the same kind of test to extract the available OOB size
based on the mtd_info and mtd_oob_ops structures.
Move this common logic into an inline function and make use of it.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Suggested-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>