metadata: extend writes to zero space

metadata: extend writes to zero space

Previously, consecutive copies of metadata would have garbage
data in the space between them.  After metadata wrapping,
the garbage would be portions of old metadata.  This made
analysis of the metadata area more difficult.

This would happen because the start of new copy of metadata
is advanced from the end of the last copy to start at the
next 512 byte boundary.

Zero the space between consecutive copies of metadata by
extending each metadata write to end at the next 512 byte
boundary.  The size of the metadata itself is not extended,
only the write.  The buffer being written contains the
metadata text followed by the necessary number of zeros.