file-posix: check the use_lock before setting the file lock

The scenario is that when accessing a volume on an NFS filesystem
without supporting the file lock,  Qemu will complain "Failed to lock
byte 100", even when setting the file.locking = off.

We should do file lock related operations only when the file.locking is
enabled, otherwise, the syscall of 'fcntl' will return non-zero.

Signed-off-by: Li Feng <fengli@smartx.com>
Message-Id: <1607341446-85506-1-git-send-email-fengli@smartx.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>