[Bug] SingleDirectoryDbLedgerStorage#flushMutex does not release lock on all exception paths

[StevenLuMT]

BUG REPORT

Describe the bug

Because this pr(#3772) was merged,SingleDirectoryDbLedgerStorage#flushMutex does not release lock on all exception paths

To Reproduce

Steps to reproduce the behavior:

1. When another pr [feature] [server] add dbStorage_readAheadCacheBatchBytesSize properties when read ahead entries #3895 went to modify SingleDirectoryDbLedgerStorage.java, the checkstyle workflow run failed for
   Error: Medium: org.apache.bookkeeper.bookie.storage.ldb.SingleDirectoryDbLedgerStorage.checkpoint(CheckpointSource$Checkpoint) does not release lock on all exception paths [org.apache.bookkeeper.bookie.storage.ldb.SingleDirectoryDbLedgerStorage] At SingleDirectoryDbLedgerStorage.java:[line 791] UL_UNRELEASED_LOCK_EXCEPTION_PATH

Expected behavior
the checkstyle workflow run successfully

Screenshots

Additional context
old code:

in #3772 updated