udp: Consistently use -1 to indicate un-opened sockets in maps

udp uses the udp_tap_map, udp_splice_ns and udp_splice_init tables to keep
track of already opened sockets bound to specific ports.  We need a way to
indicate entries where a socket hasn't been opened, but the code isn't
consistent if this is indicated by a 0 or a -1:
  * udp_splice_sendfrom() and udp_tap_handler() assume that 0 indicates
    an unopened socket
  * udp_sock_init() fills in -1 for a failure to open a socket
  * udp_timer_one() is somewhere in between, treating only strictly
    positive fds as valid

-1 (or, at least, negative) is really the correct choice here, since 0 is
a theoretically valid fd value (if very unlikely in practice).  Change to
use that consistently throughout.

The table does need to be initialised to all -1 values before any calls to
udp_sock_init() which can happen from conf_ports().  Because C doesn't make
it easy to statically initialise non zero values in large tables, this does
require a somewhat awkward call to initialise the table from conf().  This
is the best approach I could see for the short term, with any luck it will
go away at some point when those socket tables are replaced by a unified
flow table.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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