ubi: fastmap: Implement produce_free_peb()

If fastmap requests a free PEB for a pool and UBI is busy
with erasing PEBs we need to offer a function to wait for one.
We can reuse produce_free_peb() from the non-fastmap WL code
but with different locking semantics.

Cc: stable@vger.kernel.org # 4.1.x-
Reported-and-tested-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Richard Weinberger <richard@nod.at>