libxfs: port list_cmp_func_t to userspace

Synchronize our list_sort ABI to match the kernel's.  This will make it
easier to port the log item precommit sorting code to userspace as-is in
the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>