fuse: add basic infrastructure to support idmappings

fuse: add basic infrastructure to support idmappings

Add some preparational changes in fuse_get_req/fuse_force_creds
to handle idmappings.

Miklos suggested [1], [2] to change the meaning of in.h.uid/in.h.gid
fields when daemon declares support for idmapped mounts. In a new semantic,
we fill uid/gid values in fuse header with a id-mapped caller uid/gid (for
requests which create new inodes), for all the rest cases we just send -1
to userspace.

No functional changes intended.

Link: https://lore.kernel.org/all/CAJfpegsVY97_5mHSc06mSw79FehFWtoXT=hhTUK_E-Yhr7OAuQ@mail.gmail.com/
Link: https://lore.kernel.org/all/CAJfpegtHQsEUuFq1k4ZbTD3E1h-GsrN3PWyv7X8cg6sfU_W2Yw@mail.gmail.com/
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>