ipv4: Fix a data-race around sysctl_fib_sync_mem.

ipv4: Fix a data-race around sysctl_fib_sync_mem.

BugLink: https://bugs.launchpad.net/bugs/1988351
[ Upstream commit 73318c4b7dbd0e781aaababff17376b2894745c0 ]

While reading sysctl_fib_sync_mem, it can be changed concurrently.
So, we need to add READ_ONCE() to avoid a data-race.

Fixes: 9ab948a91b2c ("ipv4: Allow amount of dirty memory from fib resizing to be controllable")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>