Unlock LFC mutex when LFC cache is disabled (#10235)

## Problem

See https://github.com/neondatabase/neon/issues/10233
`lfc_containsv` returns with holding lock when LFC was disabled.
This bug was introduced in commit  78938d1b59

## Summary of changes

Release lock before return.

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon/file_cache.c

@@ -541,6 +541,7 @@ lfc_cache_containsv(NRelFileInfo rinfo, ForkNumber forkNum, BlockNumber blkno,
 		}
 		else
 		{
+			LWLockRelease(lfc_lock);
 			return found;
 		}