libceph: don't reset kvec in prepare_write_banner()

(cherry picked from commit d329156f16306449c273002486c28de3ddddfd89)

Move the kvec reset for a connection out of prepare_write_banner and
into its only caller.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>