shadow: fix default value in SHA_get_salt_rounds()

Backport a patch [1] to fix chpasswd, gpasswd and passwd "hang" for
several minutes (10-20min) at 100% cpu usage though they finally
terminate successfully.

[1] https://github.com/shadow-maint/shadow/issues/393



Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>