client: add function to check if file handle exists

It is named _ll_fh_exists meaning it is a helper func
only to be used with low level APIs.

Fixes: https://tracker.ceph.com/issues/64313
Signed-off-by: Dhairya Parmar <[email protected]>

Author: dparmar18

src/client/Client.h

@@ -1026,6 +1026,9 @@ class Client : public Dispatcher, public md_config_obs_t {
     return it->second;
   }
   int get_fd_inode(int fd, InodeRef *in);
+  bool _ll_fh_exists(Fh *f) {
+    return ll_unclosed_fh_set.count(f);
+  }
 
   // helpers
   void wake_up_session_caps(MetaSession *s, bool reconnect);