Revert "net: limit tcp/udp rmem/wmem to SOCK_{RCV,SND}BUF_MIN"

Commit 8133534c ("net: limit tcp/udp rmem/wmem to
SOCK_{RCV,SND}BUF_MIN") modified four sysctls to enforce that the values
written to them are not less than SOCK_MIN_{RCV,SND}BUF.

That change causes 4096 to no longer be accepted as a valid value for
'min' in tcp_wmem and udp_wmem_min. 4096 has been the default for both
of those sysctls for a long time, and unfortunately seems to be an
extremely popular setting. This change breaks a large number of sysctl
configurations at Facebook.

That commit referred to b1cb59cf ("net: sysctl_net_core: check
SNDBUF and RCVBUF for min length"), which choose to use the SOCK_MIN
constants as the lower limits to avoid nasty bugs. But AFAICS, a limit
of SOCK_MIN_SNDBUF isn't necessary to do that: the BUG_ON cited in the
commit message seems to have happened because unix_stream_sendmsg()
expects a minimum of a full page (ie SK_MEM_QUANTUM) and the math broke,
not because it had less than SOCK_MIN_SNDBUF allocated.

This particular issue doesn't seem to affect TCP however: using a
setting of "1 1 1" for tcp_{r,w}mem works, although it's obviously
suboptimal. SK_MEM_QUANTUM would be a nice minimum, but it's 64K on
some archs, so there would still be breakage.

Since a value of one doesn't seem to cause any problems, we can drop the
minimum 8133534c added to fix this.

This reverts commit 8133534c.

Fixes: 8133534c ("net: limit tcp/udp rmem/wmem to SOCK_MIN...")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Sorin Dumitru <sorin@returnze.ro>
Signed-off-by: Calvin Owens <calvinowens@fb.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>