Revert "firmware_loader: Block path traversal"

This reverts commit f0e5311a.

Reason for revert: The logic that allows the kernel to load firmware
requires the path to originate in /lib/firmware, but ChromeOS ships
some peripheral firmware in DLCs at /run/imageloader. Upstream linux
does not have any need to load firmware files from anywhere other than
/lib/firmware, and so they have landed a change that prevents firmware
loading from any other location, breaking ChromeOS firmware loading
from DLC, for instance on the FM350 modem. This CL reverts the
upstream patch, to fix the regression in ChromeOS.

This revert is only necessary on FM350 up to R134, as the FM350 modem
will not load its firmware through the kernel on newer milestones.

BUG=b:389154359,b:383128339
TEST=Verified FM350 firmware can be loaded from DLC on R134

Change-Id: I5c117f35da23a802e0b03187c5dfd8fcc2a793f2
Signed-off-by: Daniel Winkler <danielwinkler@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/6169026


Reviewed-by: Aleksander Morgado <aleksandermj@google.com>
Signed-off-by: Hubert Mazur <hmazur@google.com>