xfs: simplify xfs_zero_remaining_bytes

xfs_zero_remaining_bytes() open codes a log of buffer manupulations
to do a read forllowed by a write. It can simply be replaced by an
uncached read followed by a xfs_bwrite() call.

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