Most "production" netlink clients use large buffers to
make dump efficient, which means that handling of dump
continuation in the kernel is not very well tested.

Add an option for debugging / testing handling of dumps.
It enables printing of extra netlink-level debug and
lowers the recv() buffer size in one go. When used
without any argument (--dbg-small-recv) it picks
a very small default (4000), explicit size can be set,
too (--dbg-small-recv 5000).

Example:

$ ./cli.py [...] --dbg-small-recv
Recv: read 3712 bytes, 29 messages
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
 [...]
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
Recv: read 3968 bytes, 31 messages
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
 [...]
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
Recv: read 532 bytes, 5 messages
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
 [...]
   nl_len = 128 (112) nl_flags = 0x0 nl_type = 19
   nl_len = 20 (4) nl_flags = 0x2 nl_type = 3

(the [...] are edits to shorten the commit message).

Note that the first message of the dump is sized conservatively
by the kernel.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>