nfsd4: adjust buflen for encoded attrs bitmap based on actual bitmap length

The existing code adjusted it based on the worst case scenario for the returned
bitmap and the best case scenario for the supported attrs attribute.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[bfields@redhat.com: removed likely/unlikely's]
Signed-off-by: J. Bruce Fields <bfields@redhat.com>