ceph: stash idmapping in mdsc request

When sending a mds request cephfs will send relevant data for the
requested operation. For creation requests the caller's fs{g,u}id is
used to set the ownership of the newly created filesystem object. For
setattr requests the caller can pass in arbitrary {g,u}id values to
which the relevant filesystem object is supposed to be changed.

If the caller is performing the relevant operation via an idmapped mount
cephfs simply needs to take the idmapping into account when it sends the
relevant mds request.

In order to support idmapped mounts for cephfs we stash the idmapping
whenever they are relevant for the operation for the duration of the
request. Since mds requests can be queued and performed asynchronously
we make sure to keep the idmapping around and release it once the
request has finished.

In follow-up patches we will use this to send correct ownership
information over the wire. This patch just adds the basic infrastructure
to keep the idmapping around. The actual conversion patches are all
fairly minimal.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>