afs: Avoid needless W-locks for afs_FindVCache

afs: Avoid needless W-locks for afs_FindVCache

The callers of afs_FindVCache must hold at least a read lock on
afs_xvcache; some hold a shared or write lock (and set IS_SLOCK or
IS_WLOCK in the given flags). Two callers (afs_EvalFakeStat_int and
afs_DoBulkStat) currently hold a write lock, but neither of them need
to.

In the optimal case, where afs_FindVCache finds the given vcache, this
means that we unnecessarily hold a write lock on afs_xvcache. This can
impact performance, since afs_xvcache can be a very frequently
accessed lock (a simple operation like afs_PutVCache briefly holds a
read lock, for example).

To avoid this, have afs_DoBulkStat hold a shared lock on afs_xvcache,
upgrading to a write lock when needed. afs_EvalFakeStat_int doesn't
ever need a write lock at all, so just convert it to a read lock.

Change-Id: I5bd58b9e3a577c9e1ebf1bc3719e65a6c0af5cb8
Reviewed-on: https://gerrit.openafs.org/12656
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>