xfs_copy: genericize write helper to facilitate separate log buf

xfs_copy: genericize write helper to facilitate separate log buf

xfs_copy uses a fixed write buffer throughout the code for the various
targets. This is a global buffer and is assumed to be the write source
in the do_write() helper used by the log clearing code.

v5 superblock log formatting will require a larger, independent buffer
to format the log. Therefore, update do_write() to accept a write buffer
parameter to optionally override the global write buffer. This patch
does not change current behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>