UPSTREAM: erofs-utils: make liberofs more C++ friendly

1. Add extern "C" wrappers to headers, so that they can be included
   from C++
2. Add const keywords to certain pointer type params

Link: https://lore.kernel.org/r/20211201135315.3732-1-xiang@kernel.org
Signed-off-by: Kelvin Zhang <zhangkelvin@google.com>
Signed-off-by: Gao Xiang <xiang@kernel.org>
(cherry picked from commit 519dbd2368240c4c6d159459ab5614e1d9540c3c)
Change-Id: I43522f84586e855f06a5d5c626e6a476c1c39aa4
26 files changed