nfsd4: decrease nfsd4_encode_fattr stack usage

A struct svc_fh is 320 bytes on x86_64, it'd be better not to have these
on the stack.

kmalloc'ing them probably isn't ideal either, but this is the simplest
thing to do.  If it turns out to be a problem in the readdir case then
we could add a svc_fh to nfsd4_readdir and pass that in.

Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>