ANDROID: regression introduced override_creds=off

Solve a regression introduced by
commit f5f87eca
("ANDROID: overlayfs: override_creds=off option bypass creator_cred")
where a crash is observed a crash in ovl_create_or_link() when a
simple re-direction command in vendor directory.

/vendor/bin/<Any test> > /vendor/bin/test_log.txt 2>&amp;1&

After further debugging we see that if the output is redirected to a
file which doesn’t exist we see this stack:

[  377.382745]  ovl_create_or_link+0xac/0x710
[  377.382745]  ovl_create_object+0xb8/0x110
[  377.382745]  ovl_create+0x34/0x40
[  377.382745]  path_openat+0xd44/0x15a8
[  377.382745]  do_filp_open+0x80/0x128
[  377.382745]  do_sys_open+0x140/0x250
[  377.382745]  __arm64_sys_openat+0x2c/0x38

ovl_override_creds returns NULL because the override_cred flag is set
to false.  This causes ovl_revert_creds also to fail.

There is another call to check override_cred in override_cred call
which overrides the creds permanently as there no revert_creds
associated.  So whenever next commit_cred is called we see the crash
as the credentials are permanently overridden.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Tested-by: Rishabh/Jeevan <jshriram@qualcomm.corp-partner.google.com>
Bug: 140816499
Change-Id: Icd0d9be82fc57af5ead1eeab99f79adf3adf62ef