nds32: Fix xfs_buf built failed by export invalidate_kernel_vmap_range and flush_kernel_vmap_range

It broke the 'allmodconfig' build.
fs/xfs/xfs_buf.c: In function 'xfs_buf_bio_end_io':
fs/xfs/xfs_buf.c:1242:3: error: implicit declaration of function 'invalidate_kernel_vmap_range' [-Werror=implicit-function-declaration]
   invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/xfs/xfs_buf.c: In function 'xfs_buf_ioapply_map':
fs/xfs/xfs_buf.c:1312:4: error: implicit declaration of function 'flush_kernel_vmap_range' [-Werror=implicit-function-declaration]
    flush_kernel_vmap_range(bp->b_addr,
    ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>