vlog: Fix a deadlock bug.

vlog: Fix a deadlock bug.

Calling VLOG_FATAL() while holding the 'log_file_mutex" may lead to
deadlock since VLOG_FATAL() implementation tries to acquire the
same lock. Fix this by building the error message first, then
call VLOG_FATAL() after the 'log_file_mutex' has been released.

This bug is not likely show up in practice since chown() usually
won't fail. It is still better to have a correct implementation.

Reported-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>