[OpenAFS-port-freebsd] Next build breaker on 9.1

[Garrett Wollman]

/home/buildbot/openafs/src/afs/FBSD/osi_file.c: In function 'afs_osi_Read':
/home/buildbot/openafs/src/afs/FBSD/osi_file.c:180: warning: passing argument 10 of 'vn_rdwr' from incompatible pointer type
/home/buildbot/openafs/src/afs/FBSD/osi_file.c: In function 'afs_osi_Write':
/home/buildbot/openafs/src/afs/FBSD/osi_file.c:212: warning: passing argument 10 of 'vn_rdwr' from incompatible pointer type

This is because vn_rdwr now needs an (ssize_t *) rather than an (int
*).  Unfortunately, there was no __FreeBSD_version bump.  The change
happened in 9-stable at r233353 (between 900505 and 900506), and in
10-current at r231949 (between 1000008 and 1000009).

Unfortunately I don't have the time to test and submit a patch for
this right now.

-GAWollman