UBUNTU: SAUCE: shiftfs: Correct id translation for lower fs operations

UBUNTU: SAUCE: shiftfs: Correct id translation for lower fs operations

BugLink: https://bugs.launchpad.net/bugs/1850867
Several locations which shift ids translate user/group ids before
performing operations in the lower filesystem are translating
them into init_user_ns, whereas they should be translated into
the s_user_ns for the lower filesystem. This will result in using
ids other than the intended ones in the lower fs, which will
likely not map into the shifts s_user_ns.

Change these sites to use shift_k[ug]id() to do a translation
into the s_user_ns of the lower filesystem.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
CVE-2019-15793

Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>