udhcpd: code shrink

udhcpd: code shrink

function                                             old     new   delta
is_nip_reserved_as_static                              -      28     +28
get_static_nip_by_mac                                 43      47      +4
udhcpd_main                                         1459    1454      -5
send_offer                                           449     444      -5
read_leases                                          309     299     -10
is_nip_reserved                                       20       -     -20
packed_usage                                       33283   33243     -40
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/4 up/down: 32/-80)            Total: -48 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>