fs: ubifs: Add volume mounted check

Safety guard in the U-Boot filesystem glue code, because these functions
are called from different parts of the codebase.  For generic filesystem
handling this should have been checked in blk_get_device_part_str()
already.  Commands from cmd/ubifs.c should also check this before
calling those functions, but you never know?!

Signed-off-by: Alexander Dahl <ada@thorsis.com>