exec: Check for a pending fatal signal instead of core_state

Prevent exec continuing when a fatal signal is pending by replacing
mmap_read_lock with mmap_read_lock_killable.  This is always the right
thing to do as userspace will never observe an exec complete when
there is a fatal signal pending.

With that change it becomes unnecessary to explicitly test for a core
dump in progress.  In coredump_wait zap_threads arranges under
mmap_write_lock for all tasks that use a mm to also have SIGKILL
pending, which means mmap_read_lock_killable will always return -EINTR
when old_mm->core_state is present.

Link: https://lkml.kernel.org/r/87fstux27w.fsf@disp2133
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>