ceph: messenger: use read_partial() in read_partial_message()

(cherry picked from commit 57dac9d1620942608306d8c17c98a9d1568ffdf4)

There are two blocks of code in read_partial_message()--those that
read the header and footer of the message--that can be replaced by a
call to read_partial().  Do that.

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