libxfs: split out xfs->libxfs mappings

libxfs: split out xfs->libxfs mappings

The defines that map the external libxfs namespace are found only in
libxfs_priv.h. That means we have to re-declare all the exported
function prototypes in libxfs.h so that external uses know about
them and can use them. This also means we effectively have duplicate
function prototypes as they are all already declared in the xfs_*
namespace due to the includes of the libxfs header files through
libxfs.h.

Split the mapping macros out from libxfs_priv.h into a separate
libxfs_api_defs.h and include that header file directly in both
libxfs-priv.h and libxfs.h before we include any other header file.
This means that all the xfs_* namespace definitions are mapped to
libxfs_* namespaces correctly and we don't need to have duplicate
prototypes.

This also points out all the function prototypes the external code
uses but does not have function prototypes exposed by the mapped
libxfs header files, and hence indicates future kernel/user libxfs
synchronisation work that needs to be done.

Signed-off-by: Dave Chinner <dchinner@redhat.com>