NFSD: Avoid multiple -Wflex-array-member-not-at-end warnings

NFSD: Avoid multiple -Wflex-array-member-not-at-end warnings

Replace flexible-array member with a fixed-size array.

With this changes, fix many instances of the following type of
warnings:

fs/nfsd/nfsfh.h:79:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/state.h:763:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/state.h:669:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/state.h:549:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/xdr4.h:705:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/xdr4.h:678:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>