core: RPMB_FS: remember owner uuid

Prior to this patch was the owning uuid looked up via the current
session.  The assumption that the uuid of the context of the current
session is the same as the owner of a RPMB file doesn't hold any longer
after:
078f18f ("core: RPMB FS: provide tee_rpmb_fs_raw_open()")
was introduced.

Instead the pointer to the uuid passed as the owner is stored in the
file handle and used as needed when encrypting/decrypting the file.

Fixes: #1780
Tested-by: Jens Wiklander <[email protected]> (QEMU)
Signed-off-by: Jens Wiklander <[email protected]>