libxfs: Replace XFS_BUF_SET_PTR with xfs_buf_associate_memory

libxfs: Replace XFS_BUF_SET_PTR with xfs_buf_associate_memory

We test the return value of the macro, but it returns
returns a side-effect which looks like failure.  Write
a userspace-libxfs-specific version of xfs_buf_associate_memory
to make this code a tad more like the kernel, with a proper
return value to boot.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>