msg: do not const methods that return a mutable pointer

Add remove misleading const on post_rx_buffer argument.

http://tracker.ceph.com/issues/10213 Fixes: #10213

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #3050 from dachary/wip-10213-const

msg: do not const methods that return a mutable pointer

Reviewed-by: Greg Farnum <gfarnum@redhat.com>