UBI: Fix invalidate_fastmap()

Onging tests uncovered that invalidate_fastmap() is broken.
It must not call ubi_wl_put_fm_peb() because all PEBs used
by the old fastmap have already been put back.

Change-Id: Icd4249cb30c57e5cae197e1125d84f1a2c04fc20
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Git-commit: 8930fa500f55215f0211db9b78255e727ac3f282
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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