ueventd: Increase uevent buffer size to 2MB

Let's increase the receive buffer size for the NETLINK_KOBJECT_UEVENT
socket to 2MB. Unless a large number of uevents queue up, that memory is
not allocated anyways. The receive buffer size serves only as an upper
limit for the total amount of memory consumed by all skbs queued to a
specific socket.

We experienced situations where ueventd got blocked for multiple seconds
while writing to /dev/kmsg, and the receive buffer overflowed in the
meantime.

Test: compile
Bug: 72648767
Change-Id: Ice6d7b9c5ed9c83efbad6111086ce95ac6432561