tcp: Check for filled TCP option space before SACK

Update the SACK check to work with zero option space available, a case
that's possible with MPTCP but not MD5+TS. Maintained only one
conditional branch for insufficient SACK space.

v1 -> v2:
- Moves the check inside the SACK branch by taking recent SACK fix:

    9424e2e7 (tcp: md5: fix potential overestimation of TCP option space)

  in to account, but modifies it to work in MPTCP scenarios beyond the
  MD5+TS corner case.

Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>