(cherry picked from commit 739c905b)

Move the code that prepares to write the data portion of a message
into its own function.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>