net: tcp: Reduce size of `state` member from 32 to 4 bits

No more than 4 bits are necessary to store the state of a TCP connection,
so better pack it using bitfields so that it uses only 4 bits instead of
32, by sharing space with `retry_timeout_shift` and `flags` fields.

There are 12 (or 14, if you count the 2 unused bits in the `flags`
field) bits remaining in the same dword, but I don't know what to to
stuff there yet.

This also changes all direct field access for the `state` field to
function calls.  These functions are provided as `static inline`
functions and they perform only casts, so there's no function call
overhead.

Change-Id: I0197462caa0b71b287c0773ec5cd2dd4101a4766
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>