krbd: increase udev netlink socket receive buffer to 2M

krbd: increase udev netlink socket receive buffer to 2M

Even though with the previous commit we no longer block between binding
the socket and starting handling events, we still want a larger receive
buffer to accommodate for scheduling delays.  Since the filtering is
done in the listener, an estimate focused on just rbd is not accurate,
but anyway: a pair of "rbd" and "block" events for "rbd map" take 2048
bytes in the receive buffer.  This allows for roughly a thousand of
them ("rbd map" and "rbd unmap" require root and libudev makes use of
SO_RCVBUFFORCE so rmem_max limit is ignored).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 1c6cac1acaefdf59c3265d70c8d2191c59f14652)

Conflicts:
	src/krbd.cc [ trivial context ]