- fix ip route rejecting dotted quads as prefix

- fix ip route rejecting dotted quads as prefix
- adjust error message for wrong prefix not to mention address
  Previously e.g. ip route add 127.0.0.0/255.0.0.0 dev dummy0
  was rejected, saying
ip: an inet address is expected rather than "127.0.0.0/255.0.0.0"

function                                             old     new   delta
get_prefix_1                                         201     309    +108
get_prefix                                            55      73     +18
get_addr                                              55      73     +18
get_addr32                                            48      58     +10
get_addr_1                                           249     204     -45
.rodata                                           114569  114524     -45
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/2 up/down: 154/-90)            Total: 64 bytes