mtd: ubi: Free peb's synchronously for fastmap

At first mount it's possible that there are not enough free PEBs since
there are PEB's pending to be erased. In such scenario, fm_pool (which is
the pool from which user required PEBs are allocated) will be empty.
Try fixing the above described situation by synchronously performing
pending erase work, thus produce another free PEB.

Change-Id: If4152ed8db8123a93c2c4fd4bad694a5b0079c6c
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>