vol: Make FindLinkHandle static and namei-only

FindLinkHandle is only referenced inside vol-salvage.c. Also, the
concept of a link table only exists on namei, so the function is only
used for the namei server (and it's only called by other namei-only
code).

So, make the function static, and put it inside the AFS_NAMEI_ENV
ifdef, to be a little more clear about where it can be used. Moving it
inside the AFS_NAMEI_ENV ifdef also avoids a warning if FindLinkHandle
is made static, since otherwise the function would be defined but
unused on non-namei.

This change should incur no difference in behavior; it is just code
reorganization.

Change-Id: Ia8cdadafaf15c724462f600514aa59910619a090
Reviewed-on: http://gerrit.openafs.org/10848
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>