rename variable to avoid shadowing (#5204)

Summary:
this PR fixes the following compile warning:
```
db/memtable.cc: In member function ‘virtual void rocksdb::MemTableIterator::Seek(const rocksdb::Slice&)’:
db/memtable.cc:321:22: error: declaration of ‘user_key’ shadows a member of 'this' [-Werror=shadow]
       Slice user_key(ExtractUserKey(k));
                      ^
db/memtable.cc: In member function ‘virtual void rocksdb::MemTableIterator::SeekForPrev(const rocksdb::Slice&)’:
db/memtable.cc:338:22: error: declaration of ‘user_key’ shadows a member of 'this' [-Werror=shadow]
       Slice user_key(ExtractUserKey(k));
                      ^
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5204

Differential Revision: D14970160

Pulled By: miasantreble

fbshipit-source-id: 388eb089f90c4528cc6d615dd4607fb53ceac705

db/memtable.cc

@@ -318,9 +318,9 @@ class MemTableIterator : public InternalIterator {
     PERF_COUNTER_ADD(seek_on_memtable_count, 1);
     if (bloom_) {
       // iterator should only use prefix bloom filter
-      Slice user_key(ExtractUserKey(k));
-      if (prefix_extractor_->InDomain(user_key) &&
-          !bloom_->MayContain(prefix_extractor_->Transform(user_key))) {
+      Slice user_k(ExtractUserKey(k));
+      if (prefix_extractor_->InDomain(user_k) &&
+          !bloom_->MayContain(prefix_extractor_->Transform(user_k))) {
         PERF_COUNTER_ADD(bloom_memtable_miss_count, 1);
         valid_ = false;
         return;
@@ -335,9 +335,9 @@ class MemTableIterator : public InternalIterator {
     PERF_TIMER_GUARD(seek_on_memtable_time);
     PERF_COUNTER_ADD(seek_on_memtable_count, 1);
     if (bloom_) {
-      Slice user_key(ExtractUserKey(k));
-      if (prefix_extractor_->InDomain(user_key) &&
-          !bloom_->MayContain(prefix_extractor_->Transform(user_key))) {
+      Slice user_k(ExtractUserKey(k));
+      if (prefix_extractor_->InDomain(user_k) &&
+          !bloom_->MayContain(prefix_extractor_->Transform(user_k))) {
         PERF_COUNTER_ADD(bloom_memtable_miss_count, 1);
         valid_ = false;
         return;