coredump: initialize coredump_size in submit_coredump() (#4219)

coredump: initialize coredump_size in submit_coredump() (#4219)

If ulimit is smaller than page_size(), function save_external_coredump()
returns -EBADSLT and this causes skipping whole core dumping part in
submit_coredump(). Initializing coredump_size to UINT64_MAX prevents
evaluating a condition with uninitialized varialbe which leads to
calling allocate_journal_field() with coredump_fd = -1 which causes
aborting.

Signed-off-by: Matej Habrnal <mhabrnal@redhat.com>