namei: Calculate offset before use in GetFreeTag

namei: Calculate offset before use in GetFreeTag

We pass 'offset' to FDH_LOCKFILE here, but 'offset' is calculated a
few lines below. The compiler doesn't catch this, since FDH_LOCKFILE
is a macro that ignores the second argument (except on WINNT).

Just move the calculation a few lines above, to ensure 'offset' is set
before it's actually used, to avoid issues if and when FDH_LOCKFILE is
changed to use the second argument.

Change-Id: If858005cb83370ca905f93f9d3d7eed2941ddf99
Reviewed-on: https://gerrit.openafs.org/14654
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>