metadump: sanitise write_buf/index return values

metadump: sanitise write_buf/index return values

Write_buf/write_index use confusing boolean values for return,
meaning that it's hard to tell what the correct error return is
supposed to be.  Convert them to return zero on success or a
negative errno otherwise so that it's clear what the error case is.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>