xfs: fix memory leak in xfs_iext_free_last_leaf

found the issue by kmemleak.
unreferenced object 0xffff8800674611c0 (size 16):
    xfs_iext_insert+0x82a/0xa90 [xfs]
    xfs_bmap_add_extent_hole_delay+0x1e5/0x5b0 [xfs]
    xfs_bmapi_reserve_delalloc+0x483/0x530 [xfs]
    xfs_file_iomap_begin+0xac8/0xd40 [xfs]
    iomap_apply+0xb8/0x1b0
    iomap_file_buffered_write+0xac/0xe0
    xfs_file_buffered_aio_write+0x198/0x420 [xfs]
    xfs_file_write_iter+0x23f/0x2a0 [xfs]
    __vfs_write+0x23e/0x340
    vfs_write+0xe9/0x240
    SyS_write+0xa1/0x120
    do_syscall_64+0xda/0x260
Signed-off-by: NShu Wang <shuwang@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_iext_tree.c

@@ -850,9 +850,9 @@ static void
 xfs_iext_free_last_leaf(
 	struct xfs_ifork	*ifp)
 {
-	ifp->if_u1.if_root = NULL;
 	ifp->if_height--;
 	kmem_free(ifp->if_u1.if_root);
+	ifp->if_u1.if_root = NULL;
 }
 
 void