xfs: move struct xfs_bmalloca to libxfs

It no long is used for stack splits, so strip the kernel workqueue
bits from it and push it back into libxfs/xfs_bmap.h so that
it can be shared with the userspace code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>