UBI: Add some asserts to ubi_attach_fastmap()

Add more paranioa asserts to make it easier to detect
implementation errors.

Change-Id: I4493648f5a019bbec5d0046074be5b9f6abdcf59
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Git-commit: ae0d146955665411a6d4cea5e0893429cfc52807
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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