ubi: fastmap: Implement produce_free_peb()
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: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org # 4.1.x-
Reported-and-tested-by: Jörg Krause <a3c6a141b7ae4e670c91aa105ccadae5e33bda16@embedded.rocks>
Signed-off-by: Richard Weinberger <320bca71fc381a4a025636043ca86e734e31cf8b@nod.at>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs