NFSD: introduce struct nfsd_attrs

The attributes that nfsd might want to set on a file include 'struct
iattr' as well as an ACL and security label.
The latter two are passed around quite separately from the first, in
part because they are only needed for NFSv4.  This leads to some
clumsiness in the code, such as the attributes NOT being set in
nfsd_create_setattr().

We need to keep the directory locked until all attributes are set to
ensure the file is never visibile without all its attributes.  This need
combined with the inconsistent handling of attributes leads to more
clumsiness.

As a first step towards tidying this up, introduce 'struct nfsd_attrs'.
This is passed (by reference) to vfs.c functions that work with
attributes, and is assembled by the various nfs*proc functions which
call them.  As yet only iattr is included, but future patches will
expand this.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>