9p: validate PDU length

This commit adds length check for the PDU size.
The size contained in the header has to match the actual size,
except for TCP (trans_fd.c) where actual length is not known ahead
and the header's length will be checked only against the validity
range.

Link: http://lkml.kernel.org/r/20180723154404.2406-1-tomasbortoli@gmail.com


Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Reported-by:  <syzbot+65c6b72f284a39d416b4@syzkaller.appspotmail.com>
To: Eric Van Hensbergen <ericvh@gmail.com>
To: Ron Minnich <rminnich@sandia.gov>
To: Latchesar Ionkov <lucho@ionkov.net>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>