afs: Detect VIOCPREFETCH special case properly

afs: Detect VIOCPREFETCH special case properly

Currently, afs_syscall_pioctl handles the VIOCPREFETCH pioctl as a
special case, calling into a different code path to handle
backgrounding the prefetch operation. However, we detect that we're
handling a VIOCPREFETCH operation just by looking at the lower 8 bits
of the given opcode. This means that any pioctl that ends in 0x0F will
trigger this codepath, such as if we add a 'C' or 'O' pioctl that uses
code 0x0F.

We only want to catch VIOCPREFETCH requests for this code path, so fix
the check to also check if we're processing a 'V' pioctl.

Change-Id: Ica8c2364f96aa3c8b4d2213bebd9a1e4cb6fa730
Reviewed-on: https://gerrit.openafs.org/13301
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>