Revert "net: ipv4: don't let PMTU updates increase route MTU"

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

---------------------------------------

If the NIC support large mtu, tcp can use the mtu to get
higher bandwidth.

Tempory remove this patch.

This reverts commit 28d35bcd.

Signed-off-by: Weilong Chen <chenweilong@huawei.com>