Fix deadlock when KConfigIni fails to acquire a file lock

KConfigIniBackend::lock() locks a mutex, then attempts to acquire a lock
file and returns whether acquiring the lock file was successful.
However, if acquiring the lock file failed (e.g. due to a full disk), it
would not unlock the mutex but would still return false, indicating a
failure to acquire the lock to the caller. Thus, any subsequent call to
KConfigIniBackend::lock() would deadlock on the locked mutex.

I was able to reliably reproduce this by running out of disk space and
running any application (e.g. Plasma) that attempted to invoke
KConfig::sync() multiple times.