UBIFS: switch to RO mode after synchronizing

In 'ubifs_garbage_collect()' on error path, we first switch to R/O mode, and
then synchronize write-buffers (to make sure no data are lost). But the GC
write-buffer synchronization will fail, because we are already in R/O mode.
This patch re-orders this and makes sure we first synchronize the write-buffer,
and then switch to R/O mode.

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