Syzkaller found a dereference in igmp_leavegroup() where inm->inm_rti

Syzkaller found a dereference in igmp_leavegroup() where inm->inm_rti
is NULL.  It should be set in rti_fill(), but is not if malloc(9)
fails.  There is no rollback after malloc failure so the field stays
uninitialized.  The code is only called from ioctl, setsockopt or
a task.  Malloc should wait instead of failing, otherwise syscalls
would be unreliable.  While there also put an M_WAIT in the init
code.  During init malloc must not fail.
OK mvs@
Reported-by: syzbot+e22326057ccf34908d78@syzkaller.appspotmail.com