Fix macro used to check kernel_read() argument order

Fix macro used to check kernel_read() argument order

The m4 macro implementing the configure check is called
LINUX_KERNEL_READ_OFFSET_IS_LAST, but it defines a preprocessor symbol
that is just KERNEL_READ_OFFSET_IS_LAST.  Our code needs to check
for the latter being defined, not the former.

Reported by Aaron Ucko.

Change-Id: Id7cd3245b6a8eb05f83c03faee9c15bab8d0f6e8
Reviewed-on: https://gerrit.openafs.org/12808
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>