nfs: Convert to use the preferred fallthrough macro

Convert the uses of fallthrough comments to fallthrough macro. Please see
commit 294f69e6 ("compiler_attributes.h: Add 'fallthrough' pseudo
keyword for switch/case use") for detail.

Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>