ext/opcache: Log a warning when opcache lock file permissions could not be changed

[TaaviE]

Commit 9ab7982 fixed an issue where failing mkstemp would return -1 which would result in an invalid call of fchmod and the clobbering of errno. But the fchmod call itself could also fail (e.g. SELinux policy), thus the return code should also be checked and a relevant warning logged.

[nielsdos]

Merged into master, thanks!