Merge pull request #25422 from rzarzynski/wip-bl-drop-at_buffer_headtail

common: drop at_buffer_{head,tail} from buffer::ptr.

Reviewed-by: Josh Durgin <jdurgin@redhat.com>

diff --cc src/common/buffer.cc
Simple merge

diff --cc src/include/buffer.h
Simple merge

diff --cc src/test/bufferlist.cc
Simple merge