rgw/posix: name the lock_guard in BucketCacheEntry::reclaim()

resolves compiler warning:

rgw/driver/posix/bucket_cache.h: In instantiation of ‘bool file::listing::BucketCacheEntry<D, B>::reclaim(const cohort::lru::ObjectFactory*) [with D = rgw::sal::POSIXDriver; B = rgw::sal::POSIXBucket]’:
src/rgw/driver/posix/bucket_cache.h:139:8:   required from here
  139 |   bool reclaim(const cohort::lru::ObjectFactory* newobj_fac) {
      |        ^~~~~~~
src/rgw/driver/posix/bucket_cache.h:147:7: warning: ignoring return value of ‘std::lock_guard<_Mutex>::lock_guard(mutex_type&) [with _Mutex = std::mutex; mutex_type = std::mutex]’, declared with attribute ‘nodiscard’ [-Wunused-result]
  147 |       lock_guard{mtx};
      |       ^~~~~~~~~~

Signed-off-by: Casey Bodley <[email protected]>

Author: cbodley

src/rgw/driver/posix/bucket_cache.h

@@ -144,7 +144,7 @@ struct BucketCacheEntry : public cohort::lru::Object
     { /* anon block */
       /* in this case, we are being called from a context which holds
        * A partition lock, and this may be still in use */
-      lock_guard{mtx};
+      auto lock = lock_guard{mtx};
       if (! deleted()) {
 	flags |= FLAG_DELETED;
 	bc->recycle_count++;