coda: potential buffer overflow in coda_psdev_write()

coda: potential buffer overflow in coda_psdev_write()

Add checks to make sure the downcall message we got from the Coda cache
manager is large enough to contain the data it is supposed to have.
i.e.  when we get a CODA_ZAPDIR we can access &out->coda_zapdir.CodaFid.

Link: http://lkml.kernel.org/r/894fb6b250add09e4e3935f14649f21284a5cb18.1558117389.git.jaharkes@cs.cmu.edu
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>