kdbus: Do not use fstat() to determinte memfd size

kdbus: Do not use fstat() to determinte memfd size

Memfd size is available via kdbus metadata so there is no need to use fstat.

This fixes failure on Smack system:

    [ 5352.471226] audit: type=1400 audit(946690155.785:99): lsm=SMACK fn=smack_inode_getattr action=denied subject="USER::server" object="USER::client" requested=r pid=13139 comm="gkdbus" path=2F6D656D66643A6B64627573202864656C657465642

Original idea by Insun Pyo <insun.pyo@samsung.com>

Change-Id: Ia7fe30c9604277d5ccf7cafedb0909a2c62b0d59