fs: nfs: add missing MODULE_DESCRIPTION() macros

Fix the 'make W=1' warnings:
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs_common/nfs_acl.o
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs_common/grace.o
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs/nfs.o
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs/nfsv2.o
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs/nfsv3.o
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/nfs/nfsv4.o

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>