UBIFS: assert no fixup when writing a node

The current free space fixup can result in some writing to the UBI volume
when the space_fixup flag is set.

To catch instances where UBIFS is writing to the NAND while the space_fixup
flag is set, add an assert to ubifs_write_node().

Artem: tweaked the patch, added similar assertion to the write buffer
       write path.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Reviewed-by: Matthew L. Creech <mlcreech@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>