tmpfiles: with "e" don't attempt to set permissions when file doesn't exist (#6682)

tmpfiles.d option "e" when run through systemd-tmpfiles --create should
apply configured permissions (uid,gid) only to already existing
files. When file doesn't exist we bail out with error. Instead we should
silently ignore non-existing files.

$ useradd test
$ cat /etc/tmpfiles.d/foobar.conf
e /tmp/test - test test 1d
$ ls -l /tmp/test
ls: cannot access '/tmp/test': No such file or directory

Before:
$ systemd-tmpfiles --create /etc/tmpfiles.d/foobar.conf
Adjusting owner and mode for /tmp/test failed: No such file or directory
$ echo $?
1

After:
$ systemd-tmpfiles --create /etc/tmpfiles.d/foobar.conf
$ echo $?
0

(cherry picked from commit 780e2ee1)