tipc: Allow receiving into iovec containing multiple entries

Enhances TIPC's socket receive routines to support iovec structures
containing more than a single entry. This change leverages existing
sk_buff routines to do most of the work; the only significant change
to TIPC itself is that an sk_buff now records how much data has been
already consumed as an numeric offset, rather than as a pointer to
the first unread data byte.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>