ubimkvol: check for free LEBs

ubimkvol: check for free LEBs

Check for free LEBs before trying to create a volume. This fixes
the following issue:

$ ubimkvol /dev/ubi0 -n 0 -N test_fs -m
ubimkvol: error!: cannot UBI create volume
          error 22 (Invalid argument)
Error: cannot create UBI volume

"Invalid argument" ? Not understandable. But the thing is that
-m makes ubimkvol use available space, which is zero, then it
passes zero to the ioctl, which fails in request validation
code, because 0 invalid volume size.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>