MFS: reimplement block clean marking fix
MFS' get_block() must never return a newly acquired block buffer that
is marked dirty from previous use. This patch replaces git-
dd59d50,
which assumed a working model where blocks for device NO_DEV would
never be dirty. For at least one scenario, that assumption does not
hold, triggering superblock overwrite warnings. In this patch, blocks
are explicitly marked as clean upon being repurposed. The working
model is now restored to be: the dirty state of a block is relevant
only when its associated device is not set to NO_DEV.