xfs: process free extents to busy list in FIFO order

When we're adding extents to the busy discard list, add them to the tail
of the list so that we get FIFO order.  For FITRIM commands, this means
that we send discard bios sorted in order from longest to shortest, like
we did before commit 89cfa899.

For transactions that are freeing extents, this puts them in the
transaction's busy list in FIFO order as well, which shouldn't make any
noticeable difference.

Fixes: 89cfa899 ("xfs: reduce AGF hold times during fstrim operations")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>