xfs: don't allocate the xfs_extent_busy structure for zoned RTGs

xfs: don't allocate the xfs_extent_busy structure for zoned RTGs

Busy extent tracking is primarily used to ensure that freed blocks are
not reused for data allocations before the transaction that deleted them
has been committed to stable storage, and secondarily to drive online
discard.  None of the use cases applies to zoned RTGs, as the zoned
allocator can't overwrite blocks before resetting the zone, which already
flushes out all transactions touching the RTGs.

So the busy extent tracking is not needed for zoned RTGs, and also not
called for zoned RTGs.  But somehow the code to skip allocating and
freeing the structure got lost during the zoned XFS upstreaming process.
This not only causes these structures to unnecessarily allocated, but can
also lead to memory leaks as the xg_busy_extents pointer in the
xfs_group structure is overlayed with the pointer for the linked list
of to be reset zones.

Stop allocating and freeing the structure to not pointlessly allocate
memory which is then leaked when the zone is reset.

Fixes: 080d01c41d44 ("xfs: implement zoned garbage collection")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org> # v6.15
[cem: Fix type and add stable tag]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>