1) In the full-state API when imsf_numsrc == 0
   errno should be "0", but returns EADDRNOTAVAIL

2) An illegal filter mode change
   errno should be EINVAL, but returns EADDRNOTAVAIL

3) Trying to do an any-source option without IP_ADD_MEMBERSHIP
   errno should be EINVAL, but returns EADDRNOTAVAIL

4) Adds comments for the less obvious error return values
Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>