knfsd: nfsd4: fix enc_stateid_sz for nfsd callbacks

enc_stateid_sz should be given in u32 words units, not bytes, so we were
overestimating the buffer space needed here.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>