LINUX: hold vcache while dropping dcache refs

LINUX: hold vcache while dropping dcache refs

Hold a reference on a vcache while attempting to evict the inode from
the dcache. Since the afs_xvcache lock is dropped, it could be possible
for the vcache to be flushed during this time, making it unsafe to use
the vcache after the eviction attempt.

Change-Id: I9d91db98387b7aaa986ed915420c6cafb4f12438
Reviewed-on: https://gerrit.openafs.org/12206
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@dson.org>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>