LINUX: consolidate duplicate code in osi_TryEvictDentries

LINUX: consolidate duplicate code in osi_TryEvictDentries

The two stanzas for HAVE_DCACHE_LOCK are now functionally identical;
remove the preprocessor conditionals and duplicate code.

Minor functional change is incurrred for very old (before 2.6.38) Linux
versions that have dcache_lock; we are now obtaining the d_lock as well.

This is safe because d_lock is also quite old (pre-git, 2.6.12), and it
is a spinlock that's only held for checking d_unhashed.  Therefore, it
should have negligible performance impact.  It cannot cause deadlocks or
violate locking order, because spinlocks can't be held across sleeps.

Change-Id: I08faf204e6bd82c4401cdf6048d12cd551dd18fc
Reviewed-on: https://gerrit.openafs.org/12792
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@dson.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>