net: context: Separate user data pointer from FIFO reserved space

Using the same memory as a user data pointer and FIFO reserved space
could lead to a crash in certain circumstances, those two use cases were
not completely separate.

The crash could happen for example, if an incoming TCP connection was
abruptly closed just after being established. As TCP uses the user data
to notify error condition to the upper layer, the user data pointer
could've been used while the newly allocated context could still be
waiting on the accept queue. This damaged the data area used by the FIFO
and eventually could lead to a crash.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>