UBI: Fastmap: Introduce may_reserve_for_fm()

...and kill another #ifdef.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 2f84c2469e2f2e94a45d86597baec10bfbf06847
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>