CIFS.upcall to accomodate new namespace mount opt

CIFS.upcall to accomodate new namespace mount opt

NOTE: This patch is dependent on one of the previously sent patches:
[PATCH] CIFS: New mount option for cifs.upcall namespace resolution
which introduces a new mount option called upcall_target, to
customise the upcall behaviour.

Building upon the above patch, the following patch adds functionality
to handle upcall_target as a mount option in cifs.upcall. It can have 2 values -
mount, app.
Having this new mount option allows the mount command to specify where the
upcall should happen: 'mount' for resolving the upcall to the host
namespace, and 'app' for resolving the upcall to the ns of the calling
thread. This will enable both the scenarios where the Kerberos credentials
can be found on the application namespace or the host namespace to which
just the mount operation is "delegated".
This aids use cases like Kubernetes where the mount
happens on behalf of the application in another container altogether.

Signed-off-by: Ritvik Budhiraja <rbudhiraja@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>