nfs: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly add multiple break/goto/return/fallthrough
statements instead of just letting the code fall through to the next
case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>