Revert "fib_rules: return 0 directly if an exactly same rule exists when NLM_F_EXCL...

Revert "fib_rules: return 0 directly if an exactly same rule exists when NLM_F_EXCL not supplied"

This reverts commit 68a528f9c44e29d3de7fa45ae62ee4eac0e2b2e5.

Nathan reported the new behaviour breaks Android, as Android just add
new rules and delete old ones.

If we return 0 without adding dup rules, Android will remove the new
added rules and causing system to soft-reboot.

Fixes: 68a528f9c44e ("fib_rules: return 0 directly if an exactly same rule exists when NLM_F_EXCL not supplied")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: Yaro Slav <yaro330@gmail.com>
Reported-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>