net/lwip: modify PMTU update routine when receiving RA with MTU
net/lwip: modify PMTU update routine when receiving RA with MTU

This commit modifies the codes which updates PMTU in destination cache
when Router Advertisement with MTU is received. The modified routine
will try to update PMTU on all entries in destination cache.

Change-Id: I53c0de37da7ff69257c3eaade6cdeb1f2bc27592
Signed-off-by: Wonsang Ryou <bc48fb2bc21aa3a9d3ccd13729d9fc0f5ca913dd@samsung.com>

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