ip: refactor ip4_mtrie to atomically unset leaves

ip: refactor ip4_mtrie to atomically unset leaves

ip4_mtrie set leaves atomically in set_leaf () and set_root_leaf () but
deleted leaves using regular stores in unset_leaf () and unset_root_leaf ().
Changed leaf deletion to update mtrie using atomic store-release.

Slight performance improvement was observed in benchmarking on Qualcomm
and Xeon machines. Benchmarking involved running 'ip route add' and
'ip route del' on vpp instances. Below are the routes/second for adding
and deleting 100k routes before and after the store-release changes:

Xeon Add Routes Before: 1.140e6, 1.139e6, 1.148e6, 1.158e6, 1.155e6
Xeon Add Routes After: 1.167e6, 1.170e6, 1.174e6, 1.173e6, 1.169e6
Xeon Del Routes Before: 7.287e7, 8.089e7, 6.048e7, 7.171e7, 7.821e7
Xeon Del Routes After: 8.729e7, 7.353e7, 7.856e7, 8.209e7, 7.787e7
Qualcomm Add Routes Before: 3.709e5, 3.954e5, 3.739e5, 3.759e5, 3.671e5
Qualcomm Add Routes After: 3.879e5, 3.967e5, 3.936e5, 3.764e5, 3.817e5
Qualcomm Del Routes Before: 1.286e7, 1.379e7, 1.353e7, 1.230e7, 1.331e7
Qualcomm Del Routes After: 1.411e7, 1.355e7, 1.373e7, 1.394e7, 1.314e7

Type: refactor

Change-Id: If3acd25a2fb87addd0eb13d82d3c8f46579e8060
Signed-off-by: Jason Zhang <jason.zhang2@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>